ln/test: add multi-purpose trampoline test helper

diff --git a/lightning/src/ln/blinded_payment_tests.rs b/lightning/src/ln/blinded_payment_tests.rs
index d62f799..621c510 100644
--- a/lightning/src/ln/blinded_payment_tests.rs
+++ b/lightning/src/ln/blinded_payment_tests.rs
@@ -2428,50 +2428,6 @@ fn test_trampoline_blinded_receive() {
 	do_test_trampoline_relay(true, TrampolineTestCase::OuterCLTVLessThanTrampoline);
 }
 
-/// Creates a blinded tail where Carol receives via a blinded path.
-fn create_blinded_tail(
-	secp_ctx: &Secp256k1<All>, override_random_bytes: [u8; 32], carol_node_id: PublicKey,
-	carol_auth_key: ReceiveAuthKey, trampoline_cltv_expiry_delta: u32,
-	excess_final_cltv_delta: u32, final_value_msat: u64, payment_secret: PaymentSecret,
-) -> BlindedTail {
-	let outer_session_priv = SecretKey::from_slice(&override_random_bytes).unwrap();
-	let trampoline_session_priv = onion_utils::compute_trampoline_session_priv(&outer_session_priv);
-
-	let carol_blinding_point = PublicKey::from_secret_key(&secp_ctx, &trampoline_session_priv);
-	let carol_blinded_hops = {
-		let payee_tlvs = ReceiveTlvs {
-			payment_secret,
-			payment_constraints: PaymentConstraints {
-				max_cltv_expiry: u32::max_value(),
-				htlc_minimum_msat: final_value_msat,
-			},
-			payment_context: PaymentContext::Bolt12Refund(Bolt12RefundContext {}),
-		}
-		.encode();
-
-		let path = [((carol_node_id, Some(carol_auth_key)), WithoutLength(&payee_tlvs))];
-
-		blinded_path::utils::construct_blinded_hops(
-			&secp_ctx,
-			path.into_iter(),
-			&trampoline_session_priv,
-		)
-	};
-
-	BlindedTail {
-		trampoline_hops: vec![TrampolineHop {
-			pubkey: carol_node_id,
-			node_features: Features::empty(),
-			fee_msat: final_value_msat,
-			cltv_expiry_delta: trampoline_cltv_expiry_delta + excess_final_cltv_delta,
-		}],
-		hops: carol_blinded_hops,
-		blinding_point: carol_blinding_point,
-		excess_final_cltv_expiry_delta: excess_final_cltv_delta,
-		final_value_msat,
-	}
-}
-
 // Creates a replacement onion that is used to produce scenarios that we don't support, specifically
 // payloads that send to unblinded receives and invalid payloads.
 fn replacement_onion(
@@ -2639,15 +2595,23 @@ fn do_test_trampoline_relay(blinded: bool, test_case: TrampolineTestCase) {
 			// Create a blinded tail where Carol is receiving. In our unblinded test cases, we'll
 			// override this anyway (with a tail sending to an unblinded receive, which LDK doesn't
 			// allow).
-			blinded_tail: Some(create_blinded_tail(
+			blinded_tail: Some(create_trampoline_forward_blinded_tail(
 				&secp_ctx,
-				override_random_bytes,
+				&nodes[2].keys_manager,
+				&[],
 				carol_node_id,
 				nodes[2].keys_manager.get_receive_auth_key(),
+				ReceiveTlvs {
+					payment_secret,
+					payment_constraints: PaymentConstraints {
+						max_cltv_expiry: u32::max_value(),
+						htlc_minimum_msat: original_amt_msat,
+					},
+					payment_context: PaymentContext::Bolt12Refund(Bolt12RefundContext {}),
+				},
 				original_trampoline_cltv,
 				excess_final_cltv,
 				original_amt_msat,
-				payment_secret,
 			)),
 		}],
 		route_params: None,
diff --git a/lightning/src/ln/functional_test_utils.rs b/lightning/src/ln/functional_test_utils.rs
index d39cee7..c192373 100644
--- a/lightning/src/ln/functional_test_utils.rs
+++ b/lightning/src/ln/functional_test_utils.rs
@@ -10,7 +10,9 @@
 //! A bunch of useful utilities for building networks of nodes and exchanging messages between
 //! nodes for functional tests.
 
-use crate::blinded_path::payment::DummyTlvs;
+use crate::blinded_path::payment::{
+	BlindedPaymentPath, DummyTlvs, ForwardNode, ReceiveTlvs, TrampolineForwardTlvs,
+};
 use crate::chain::channelmonitor::{ChannelMonitor, HTLC_FAIL_BACK_BUFFER};
 use crate::chain::transaction::OutPoint;
 use crate::chain::{BestBlock, ChannelMonitorUpdateStatus, Confirm, Listen, Watch};
@@ -40,7 +42,8 @@ use crate::ln::types::ChannelId;
 use crate::onion_message::messenger::OnionMessenger;
 use crate::routing::gossip::{NetworkGraph, NetworkUpdate, P2PGossipSync};
 use crate::routing::router::{self, PaymentParameters, Route, RouteParameters};
-use crate::sign::{EntropySource, RandomBytes};
+use crate::routing::router::{compute_fees, BlindedTail, TrampolineHop};
+use crate::sign::{EntropySource, RandomBytes, ReceiveAuthKey};
 use crate::types::features::ChannelTypeFeatures;
 use crate::types::features::InitFeatures;
 use crate::types::payment::{PaymentHash, PaymentPreimage, PaymentSecret};
@@ -5768,3 +5771,47 @@ pub fn get_scid_from_channel_id<'a, 'b, 'c>(node: &Node<'a, 'b, 'c>, channel_id:
 		.short_channel_id
 		.unwrap()
 }
+
+/// Creates a [`BlindedTail`] for a trampoline forward through a single intermediate node.
+///
+/// The resulting tail contains blinded hops built from `intermediate_nodes` plus a dummy receive
+/// TLV, with the `TrampolineHop` fee and CLTV derived from the blinded path's aggregated payinfo.
+pub fn create_trampoline_forward_blinded_tail<ES: EntropySource>(
+	secp_ctx: &bitcoin::secp256k1::Secp256k1<bitcoin::secp256k1::All>, entropy_source: ES,
+	intermediate_nodes: &[ForwardNode<TrampolineForwardTlvs>], payee_node_id: PublicKey,
+	payee_receive_key: ReceiveAuthKey, payee_tlvs: ReceiveTlvs, min_final_cltv_expiry_delta: u32,
+	excess_final_cltv_delta: u32, final_value_msat: u64,
+) -> BlindedTail {
+	let blinded_path = BlindedPaymentPath::new_for_trampoline(
+		intermediate_nodes,
+		payee_node_id,
+		payee_receive_key,
+		payee_tlvs,
+		u64::max_value(),
+		min_final_cltv_expiry_delta as u16,
+		entropy_source,
+		secp_ctx,
+	)
+	.unwrap();
+
+	BlindedTail {
+		trampoline_hops: vec![TrampolineHop {
+			pubkey: intermediate_nodes.first().map(|n| n.node_id).unwrap_or(payee_node_id),
+			node_features: types::features::Features::empty(),
+			fee_msat: compute_fees(
+				final_value_msat,
+				lightning_types::routing::RoutingFees {
+					base_msat: blinded_path.payinfo.fee_base_msat,
+					proportional_millionths: blinded_path.payinfo.fee_proportional_millionths,
+				},
+			)
+			.unwrap(),
+			cltv_expiry_delta: blinded_path.payinfo.cltv_expiry_delta as u32
+				+ excess_final_cltv_delta,
+		}],
+		hops: blinded_path.blinded_hops().to_vec(),
+		blinding_point: blinded_path.blinding_point(),
+		excess_final_cltv_expiry_delta: excess_final_cltv_delta,
+		final_value_msat,
+	}
+}
diff --git a/lightning/src/routing/router.rs b/lightning/src/routing/router.rs
index 0c0d14b..edb048c 100644
--- a/lightning/src/routing/router.rs
+++ b/lightning/src/routing/router.rs
@@ -2464,7 +2464,7 @@ impl<'a> PaymentPath<'a> {
 #[inline(always)]
 /// Calculate the fees required to route the given amount over a channel with the given fees.
 #[rustfmt::skip]
-fn compute_fees(amount_msat: u64, channel_fees: RoutingFees) -> Option<u64> {
+pub(crate) fn compute_fees(amount_msat: u64, channel_fees: RoutingFees) -> Option<u64> {
 	amount_msat.checked_mul(channel_fees.proportional_millionths as u64)
 		.and_then(|part| (channel_fees.base_msat as u64).checked_add(part / 1_000_000))
 }