sweep: add missing output to the weight estimation

diff --git a/sweep/fee_bumper.go b/sweep/fee_bumper.go
index 18d848b..a23213f 100644
--- a/sweep/fee_bumper.go
+++ b/sweep/fee_bumper.go
@@ -1668,6 +1668,16 @@ func prepareSweepTx(inputs []input.Input, changePkScript lnwallet.AddrWithKey,
 		return 0, noChange, noLocktime, err
 	}
 
+	// We also add the extra change output to the change pk scripts.
+	//
+	// NOTE: The weight estimation will not be quite accurate because the
+	// witness data is greater when overlay channels are used. But that
+	// shouldn't be a problem since we will increase the fee rate
+	// incrementally via the fee function.
+	extraChangeOut.WhenSome(func(o SweepOutput) {
+		changePkScripts = append(changePkScripts, o.TxOut.PkScript)
+	})
+
 	// Creating a weight estimator with nil outputs and zero max fee rate.
 	// We don't allow adding customized outputs in the sweeping tx, and the
 	// fee rate is already being managed before we get here.
diff --git a/sweep/fee_bumper_test.go b/sweep/fee_bumper_test.go
index fb939e7..d697f90 100644
--- a/sweep/fee_bumper_test.go
+++ b/sweep/fee_bumper_test.go
@@ -14,6 +14,7 @@ import (
 	"github.com/lightningnetwork/lnd/fn/v2"
 	"github.com/lightningnetwork/lnd/input"
 	"github.com/lightningnetwork/lnd/keychain"
+	"github.com/lightningnetwork/lnd/lntypes"
 	"github.com/lightningnetwork/lnd/lnwallet"
 	"github.com/lightningnetwork/lnd/lnwallet/chainfee"
 	"github.com/stretchr/testify/mock"
@@ -2109,3 +2110,200 @@ func createTestSpendEvent(tx *wire.MsgTx) *chainntnfs.SpendEvent {
 		Cancel: func() {},
 	}
 }
+
+// TestPrepareSweepTx tests the prepareSweepTx function behavior.
+func TestPrepareSweepTx(t *testing.T) {
+	t.Parallel()
+
+	// Create test inputs with different values.
+	inp1 := createTestInput(1000000, input.WitnessKeyHash)
+	inp2 := createTestInput(2000000, input.WitnessKeyHash)
+
+	// Test fee rate and height.
+	feeRate := chainfee.SatPerKWeight(1000)
+	currentHeight := int32(800000)
+
+	testCases := []struct {
+		name           string
+		inputs         []input.Input
+		changePkScript lnwallet.AddrWithKey
+		feeRate        chainfee.SatPerKWeight
+		currentHeight  int32
+		auxSweeper     fn.Option[AuxSweeper]
+		expectedErr    error
+		checkResults   func(t *testing.T, fee btcutil.Amount,
+			changeOuts fn.Option[[]SweepOutput],
+			locktime fn.Option[int32])
+	}{
+		{
+			name: "successful sweep with change - no " +
+				"extra output",
+			inputs:         []input.Input{&inp1, &inp2},
+			changePkScript: changePkScript,
+			feeRate:        feeRate,
+			currentHeight:  currentHeight,
+			auxSweeper:     fn.None[AuxSweeper](),
+			expectedErr:    nil,
+			checkResults: func(t *testing.T, fee btcutil.Amount,
+				changeOuts fn.Option[[]SweepOutput],
+				locktime fn.Option[int32]) {
+
+				// Calculate expected weight - only regular
+				// change output, no extra.
+				expectedWeight, err := calcSweepTxWeight(
+					[]input.Input{&inp1, &inp2},
+					[][]byte{
+						changePkScript.DeliveryAddress,
+					},
+				)
+				require.NoError(t, err)
+
+				// Expected fee based on fee rate and weight.
+				expectedFee := feeRate.FeeForWeight(
+					expectedWeight,
+				)
+
+				require.Equal(t, fee, expectedFee)
+			},
+		},
+		{
+			name:           "successful sweep with extra output",
+			inputs:         []input.Input{&inp1, &inp2},
+			changePkScript: changePkScript,
+			feeRate:        feeRate,
+			currentHeight:  currentHeight,
+			auxSweeper:     fn.Some[AuxSweeper](&MockAuxSweeper{}),
+			expectedErr:    nil,
+			checkResults: func(t *testing.T, fee btcutil.Amount,
+				changeOuts fn.Option[[]SweepOutput],
+				locktime fn.Option[int32]) {
+
+				// Calculate expected weight - includes both
+				// regular change and extra output.
+				expectedWeight, err := calcSweepTxWeight(
+					[]input.Input{&inp1, &inp2},
+					[][]byte{changePkScript.DeliveryAddress,
+						changePkScript.DeliveryAddress},
+				)
+				require.NoError(t, err)
+
+				// Expected fee based on fee rate and weight.
+				expectedFee := feeRate.FeeForWeight(
+					expectedWeight,
+				)
+
+				require.Equal(t, fee, expectedFee)
+
+				// Should have change outputs (both regular
+				// and extra).
+				require.True(t, changeOuts.IsSome())
+				outputs := changeOuts.UnwrapOr([]SweepOutput{})
+				require.Equal(t, 2, len(outputs))
+
+				// Check if extra output is present.
+				hasExtra := false
+				for _, out := range outputs {
+					if out.IsExtra {
+						hasExtra = true
+						break
+					}
+				}
+				require.True(
+					t, hasExtra, "Should have extra output",
+				)
+
+				// Locktime should be None since no inputs
+				// require locktime.
+				require.True(t, locktime.IsNone())
+			},
+		},
+		{
+			name:           "insufficient inputs",
+			inputs:         []input.Input{},
+			changePkScript: changePkScript,
+			feeRate:        feeRate,
+			currentHeight:  currentHeight,
+			auxSweeper:     fn.None[AuxSweeper](),
+			expectedErr:    ErrNotEnoughInputs,
+		},
+		{
+			name: "high fee rate causes insufficient " +
+				"inputs",
+			inputs:         []input.Input{&inp1},
+			changePkScript: changePkScript,
+			feeRate:        chainfee.SatPerKWeight(10000000),
+			currentHeight:  currentHeight,
+			auxSweeper:     fn.None[AuxSweeper](),
+			expectedErr:    ErrNotEnoughInputs,
+		},
+		{
+			name: "immature locktime",
+			inputs: []input.Input{
+				createTestInputWithLocktime(
+					1000000, input.WitnessKeyHash,
+					uint32(currentHeight+100),
+				),
+			},
+			changePkScript: changePkScript,
+			feeRate:        feeRate,
+			currentHeight:  currentHeight,
+			auxSweeper:     fn.None[AuxSweeper](),
+			expectedErr:    ErrLocktimeImmature,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			fee, changeOuts, locktime, err := prepareSweepTx(
+				tc.inputs,
+				tc.changePkScript,
+				tc.feeRate,
+				tc.currentHeight,
+				tc.auxSweeper,
+			)
+
+			// Check error expectations.
+			if tc.expectedErr != nil {
+				require.ErrorIs(t, err, tc.expectedErr)
+				return
+			}
+
+			// For successful cases, run additional checks.
+			require.NoError(t, err)
+			if tc.checkResults != nil {
+				tc.checkResults(t, fee, changeOuts, locktime)
+			}
+		})
+	}
+}
+
+// createTestInputWithLocktime creates a test input with a specific locktime
+// requirement.
+func createTestInputWithLocktime(value int64, witnessType input.WitnessType,
+	locktime uint32) *input.BaseInput {
+
+	// Create a unique test identifier based on input count.
+	hash := chainhash.Hash{}
+	hash[lntypes.HashSize-1] = byte(testInputCount.Add(1))
+
+	// Use NewCsvInputWithCltv to create an input with locktime requirement.
+	inp := input.NewCsvInputWithCltv(
+		&wire.OutPoint{
+			Hash: hash,
+		},
+		witnessType,
+		&input.SignDescriptor{
+			Output: &wire.TxOut{
+				Value: value,
+			},
+			KeyDesc: keychain.KeyDescriptor{
+				PubKey: testPubKey,
+			},
+		},
+		1, 0, locktime,
+	)
+
+	return inp
+}