actor: add tests for mailbox implementation

diff --git a/actor/mailbox_test.go b/actor/mailbox_test.go
new file mode 100644
index 0000000..b96d5e4
--- /dev/null
+++ b/actor/mailbox_test.go
@@ -0,0 +1,593 @@
+package actor
+
+import (
+	"context"
+	"sync"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+// TestMessage is a test message type that embeds BaseMessage.
+type TestMessage struct {
+	BaseMessage
+	Value int
+}
+
+// MessageType returns the type name of the message for routing/filtering.
+func (tm TestMessage) MessageType() string {
+	return "TestMessage"
+}
+
+// TestChannelMailboxSend tests the Send method of ChannelMailbox.
+func TestChannelMailboxSend(t *testing.T) {
+	t.Run("successful send", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		ctx := context.Background()
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+
+		sent := mailbox.Send(ctx, env)
+		require.True(t, sent, "Send should succeed")
+	})
+
+	t.Run("send with cancelled context", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 1)
+		// Fill the mailbox first.
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+		mailbox.TrySend(env)
+
+		ctx, cancel := context.WithCancel(context.Background())
+		// Cancel immediately.
+		cancel()
+
+		env2 := envelope[TestMessage, int]{
+			message: TestMessage{Value: 43},
+			promise: nil,
+		}
+
+		sent := mailbox.Send(ctx, env2)
+		require.False(t, sent, "Send should fail with cancelled context")
+	})
+
+	t.Run("send to closed mailbox", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		mailbox.Close()
+
+		ctx := context.Background()
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+
+		sent := mailbox.Send(ctx, env)
+		require.False(t, sent, "Send should fail on closed mailbox")
+	})
+}
+
+// TestChannelMailboxTrySend tests the TrySend method of ChannelMailbox.
+func TestChannelMailboxTrySend(t *testing.T) {
+	t.Run("successful try send", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+
+		sent := mailbox.TrySend(env)
+		require.True(t, sent, "TrySend should succeed")
+	})
+
+	t.Run("try send to full mailbox", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 1)
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+
+		// Fill the mailbox.
+		sent := mailbox.TrySend(env)
+		require.True(t, sent, "First TrySend should succeed")
+
+		// Try to send again - should fail.
+		sent = mailbox.TrySend(env)
+		require.False(t, sent, "TrySend should fail on full mailbox")
+	})
+
+	t.Run("try send to closed mailbox", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		mailbox.Close()
+
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+
+		sent := mailbox.TrySend(env)
+		require.False(t, sent, "TrySend should fail on closed mailbox")
+	})
+}
+
+// TestChannelMailboxReceive tests the Receive method of ChannelMailbox.
+func TestChannelMailboxReceive(t *testing.T) {
+	t.Run("receive messages", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		ctx := context.Background()
+
+		// Send some messages.
+		for i := 0; i < 3; i++ {
+			env := envelope[TestMessage, int]{
+				message: TestMessage{Value: i},
+				promise: nil,
+			}
+			mailbox.Send(ctx, env)
+		}
+
+		// Start receiving in a goroutine.
+		var received []int
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for env := range mailbox.Receive(ctx) {
+				received = append(received, env.message.Value)
+			}
+		}()
+
+		// Close the mailbox after sending all messages.
+		mailbox.Close()
+		wg.Wait()
+
+		require.Len(t, received, 3, "Should receive 3 messages")
+		require.Equal(t, []int{0, 1, 2}, received, "Should receive messages in order")
+	})
+
+	t.Run("receive with cancelled context", func(t *testing.T) {
+		mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+		ctx, cancel := context.WithCancel(context.Background())
+
+		// Send a message.
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+		mailbox.Send(context.Background(), env)
+
+		// Start receiving.
+		var received int
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for env := range mailbox.Receive(ctx) {
+				received++
+				_ = env
+			}
+		}()
+
+		// Cancel the context.
+		cancel()
+		wg.Wait()
+
+		// Might receive 0 or 1 message depending on timing.
+		require.LessOrEqual(t, received, 1,
+			"Should stop receiving after context cancel")
+	})
+}
+
+// TestChannelMailboxClose tests the Close and IsClosed methods.
+func TestChannelMailboxClose(t *testing.T) {
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+
+	require.False(t, mailbox.IsClosed(), "Mailbox should not be closed initially")
+
+	mailbox.Close()
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed after Close()")
+
+	// Closing again should be safe.
+	mailbox.Close()
+	require.True(t, mailbox.IsClosed(), "Mailbox should remain closed")
+}
+
+// TestChannelMailboxDrain tests the Drain method of ChannelMailbox.
+func TestChannelMailboxDrain(t *testing.T) {
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+	ctx := context.Background()
+
+	// Send some messages.
+	for i := 0; i < 3; i++ {
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: i},
+			promise: nil,
+		}
+		mailbox.Send(ctx, env)
+	}
+
+	// Close the mailbox.
+	mailbox.Close()
+
+	// Drain messages.
+	var drained []int
+	for env := range mailbox.Drain() {
+		drained = append(drained, env.message.Value)
+	}
+
+	require.Len(t, drained, 3, "Should drain 3 messages")
+	require.Equal(t, []int{0, 1, 2}, drained, "Should drain messages in order")
+}
+
+// TestChannelMailboxConcurrent tests concurrent operations on ChannelMailbox.
+func TestChannelMailboxConcurrent(t *testing.T) {
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 100)
+	ctx := context.Background()
+
+	const numSenders = 10
+	const messagesPerSender = 100
+
+	var wg sync.WaitGroup
+
+	// Start multiple senders.
+	for i := 0; i < numSenders; i++ {
+		wg.Add(1)
+		go func(senderID int) {
+			defer wg.Done()
+			for j := 0; j < messagesPerSender; j++ {
+				env := envelope[TestMessage, int]{
+					message: TestMessage{Value: senderID*1000 + j},
+					promise: nil,
+				}
+				mailbox.Send(ctx, env)
+			}
+		}(i)
+	}
+
+	// Start receiver.
+	received := make([]int, 0, numSenders*messagesPerSender)
+	var receiverWg sync.WaitGroup
+	receiverWg.Add(1)
+	go func() {
+		defer receiverWg.Done()
+		for env := range mailbox.Receive(ctx) {
+			received = append(received, env.message.Value)
+		}
+	}()
+
+	// Wait for all senders to complete.
+	wg.Wait()
+
+	// Close the mailbox now that all sends are complete.
+	mailbox.Close()
+	receiverWg.Wait()
+
+	require.Len(t, received, numSenders*messagesPerSender,
+		"Should receive all messages")
+}
+
+// TestChannelMailboxZeroCapacity tests that zero capacity defaults to 1.
+func TestChannelMailboxZeroCapacity(t *testing.T) {
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 0)
+
+	// Should default to capacity of 1.
+	env := envelope[TestMessage, int]{
+		message: TestMessage{Value: 42},
+		promise: nil,
+	}
+
+	sent := mailbox.TrySend(env)
+	require.True(t, sent, "Should be able to send one message")
+
+	// Second send should fail (mailbox full).
+	sent = mailbox.TrySend(env)
+	require.False(t, sent, "Second send should fail on full mailbox")
+}
+
+// TestChannelMailboxActorContext tests that the mailbox respects the actor's
+// context for cancellation.
+func TestChannelMailboxActorContext(t *testing.T) {
+	t.Run("send respects actor context", func(t *testing.T) {
+		actorCtx, actorCancel := context.WithCancel(context.Background())
+		mailbox := NewChannelMailbox[TestMessage, int](actorCtx, 1)
+
+		// Fill the mailbox.
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+		mailbox.TrySend(env)
+
+		// Cancel the actor context.
+		actorCancel()
+
+		// Try to send with a fresh caller context - should fail due to
+		// actor context cancellation.
+		callerCtx := context.Background()
+		env2 := envelope[TestMessage, int]{
+			message: TestMessage{Value: 43},
+			promise: nil,
+		}
+
+		sent := mailbox.Send(callerCtx, env2)
+		require.False(t, sent, "Send should fail when actor context is cancelled")
+	})
+
+	t.Run("receive respects actor context", func(t *testing.T) {
+		actorCtx, actorCancel := context.WithCancel(context.Background())
+		mailbox := NewChannelMailbox[TestMessage, int](actorCtx, 10)
+
+		// Send a message.
+		env := envelope[TestMessage, int]{
+			message: TestMessage{Value: 42},
+			promise: nil,
+		}
+		mailbox.Send(context.Background(), env)
+
+		// Start receiving with a fresh context.
+		callerCtx := context.Background()
+		var received int
+		var wg sync.WaitGroup
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			for env := range mailbox.Receive(callerCtx) {
+				received++
+				_ = env
+			}
+		}()
+
+		// Cancel the actor context.
+		actorCancel()
+		wg.Wait()
+
+		// Should have stopped receiving due to actor context cancellation.
+		require.LessOrEqual(t, received, 1,
+			"Should stop receiving when actor context is cancelled")
+	})
+}
+
+// TestMailboxConcurrentSendAndClose tests concurrent Send and Close operations
+// to ensure no race conditions or panics occur.
+func TestMailboxConcurrentSendAndClose(t *testing.T) {
+	const numSenders = 20
+	const sendsPerSender = 100
+
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 100)
+	ctx := context.Background()
+
+	var wg sync.WaitGroup
+
+	// Start receiver to drain messages.
+	var recvWg sync.WaitGroup
+	recvWg.Add(1)
+	go func() {
+		defer recvWg.Done()
+		for range mailbox.Receive(ctx) {
+			// Just drain.
+		}
+	}()
+
+	// Start multiple senders.
+	for i := 0; i < numSenders; i++ {
+		wg.Add(1)
+		go func(senderID int) {
+			defer wg.Done()
+			for j := 0; j < sendsPerSender; j++ {
+				env := envelope[TestMessage, int]{
+					message: TestMessage{Value: senderID*1000 + j},
+					promise: nil,
+				}
+				// Send may fail if mailbox closes, that's ok.
+				mailbox.Send(ctx, env)
+			}
+		}(i)
+	}
+
+	// Concurrently close the mailbox multiple times from different
+	// goroutines.
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			mailbox.Close()
+		}()
+	}
+
+	wg.Wait()
+	recvWg.Wait()
+
+	// Mailbox should be closed.
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed")
+
+	// Further sends should fail without panic.
+	env := envelope[TestMessage, int]{
+		message: TestMessage{Value: 999},
+		promise: nil,
+	}
+	sent := mailbox.Send(ctx, env)
+	require.False(t, sent, "Send should fail on closed mailbox")
+}
+
+// TestMailboxConcurrentTrySendAndClose tests concurrent TrySend and Close
+// operations to ensure no race conditions or panics occur.
+func TestMailboxConcurrentTrySendAndClose(t *testing.T) {
+	const numSenders = 20
+	const sendsPerSender = 100
+
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+
+	var wg sync.WaitGroup
+
+	// Start multiple senders using TrySend.
+	for i := 0; i < numSenders; i++ {
+		wg.Add(1)
+		go func(senderID int) {
+			defer wg.Done()
+			for j := 0; j < sendsPerSender; j++ {
+				env := envelope[TestMessage, int]{
+					message: TestMessage{Value: senderID*1000 + j},
+					promise: nil,
+				}
+				// TrySend may fail if mailbox is full or closed.
+				mailbox.TrySend(env)
+			}
+		}(i)
+	}
+
+	// Concurrently close the mailbox.
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			mailbox.Close()
+		}()
+	}
+
+	wg.Wait()
+
+	// Mailbox should be closed.
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed")
+
+	// Further sends should fail without panic.
+	env := envelope[TestMessage, int]{
+		message: TestMessage{Value: 999},
+		promise: nil,
+	}
+	sent := mailbox.TrySend(env)
+	require.False(t, sent, "TrySend should fail on closed mailbox")
+}
+
+// TestMailboxMultipleCloseCallers tests that multiple goroutines calling
+// Close() simultaneously don't cause panics or issues.
+func TestMailboxMultipleCloseCallers(t *testing.T) {
+	const numClosers = 100
+
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 10)
+
+	var wg sync.WaitGroup
+
+	// Start many goroutines all trying to close the mailbox.
+	for i := 0; i < numClosers; i++ {
+		wg.Add(1)
+		go func() {
+			defer wg.Done()
+			mailbox.Close()
+		}()
+	}
+
+	wg.Wait()
+
+	// Mailbox should be closed exactly once.
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed")
+
+	// Calling Close again should be safe.
+	mailbox.Close()
+	require.True(t, mailbox.IsClosed(), "Mailbox should remain closed")
+}
+
+// TestMailboxCloseWhileSending tests closing the mailbox while multiple
+// senders are actively sending messages.
+func TestMailboxCloseWhileSending(t *testing.T) {
+	const numSenders = 10
+	const sendsPerSender = 1000
+
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 100)
+	ctx := context.Background()
+
+	var sendWg sync.WaitGroup
+
+	// Start multiple senders.
+	for i := 0; i < numSenders; i++ {
+		sendWg.Add(1)
+		go func(senderID int) {
+			defer sendWg.Done()
+			for j := 0; j < sendsPerSender; j++ {
+				env := envelope[TestMessage, int]{
+					message: TestMessage{Value: senderID*1000 + j},
+					promise: nil,
+				}
+				// Send may fail after close, that's expected.
+				mailbox.Send(ctx, env)
+			}
+		}(i)
+	}
+
+	// Start receiver to drain messages.
+	var recvWg sync.WaitGroup
+	recvWg.Add(1)
+	receivedCount := 0
+	go func() {
+		defer recvWg.Done()
+		for range mailbox.Receive(ctx) {
+			receivedCount++
+		}
+	}()
+
+	// Close mailbox while sends are happening.
+	mailbox.Close()
+
+	sendWg.Wait()
+	recvWg.Wait()
+
+	// Should have received at least some messages (exact count depends on
+	// timing).
+	t.Logf("Received %d messages before close", receivedCount)
+
+	// Mailbox should be closed.
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed")
+}
+
+// TestMailboxStressTest performs a high-concurrency stress test with multiple
+// senders, receivers, and close operations.
+func TestMailboxStressTest(t *testing.T) {
+	const numSenders = 50
+	const numReceivers = 5
+	const sendsPerSender = 200
+
+	mailbox := NewChannelMailbox[TestMessage, int](context.Background(), 200)
+	ctx := context.Background()
+
+	var sendWg sync.WaitGroup
+
+	// Start multiple senders.
+	for i := 0; i < numSenders; i++ {
+		sendWg.Add(1)
+		go func(senderID int) {
+			defer sendWg.Done()
+			for j := 0; j < sendsPerSender; j++ {
+				env := envelope[TestMessage, int]{
+					message: TestMessage{Value: senderID*1000 + j},
+					promise: nil,
+				}
+				mailbox.Send(ctx, env)
+			}
+		}(i)
+	}
+
+	// Start multiple receivers.
+	var recvWg sync.WaitGroup
+	for i := 0; i < numReceivers; i++ {
+		recvWg.Add(1)
+		go func() {
+			defer recvWg.Done()
+			for range mailbox.Receive(ctx) {
+				// Just drain messages.
+			}
+		}()
+	}
+
+	// Wait for all sends to complete.
+	sendWg.Wait()
+
+	// Close mailbox.
+	mailbox.Close()
+
+	// Wait for all receivers to finish.
+	recvWg.Wait()
+
+	// Mailbox should be closed.
+	require.True(t, mailbox.IsClosed(), "Mailbox should be closed")
+}