Add test for dummy hop insertion

Introduces a test to verify correct handling of dummy hops
in constructed blinded paths.
Ensures that the added dummy hops are properly included and
do not interfere with the real path.

Co-authored-by: valentinewallace <[email protected]>

Author: shaavan

lightning/src/onion_message/functional_tests.rs

@@ -424,6 +424,34 @@ fn one_blinded_hop() {
 	pass_along_path(&nodes);
 }
 
+#[test]
+fn blinded_path_with_dummy() {
+	let nodes = create_nodes(2);
+	let test_msg = TestCustomMessage::Pong;
+
+	let secp_ctx = Secp256k1::new();
+	let context = MessageContext::Custom(Vec::new());
+	let entropy = &*nodes[1].entropy_source;
+	let expanded_key = ExpandedKey::new([42; 32]);
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
+		&[],
+		5,
+		nodes[1].node_id,
+		context,
+		entropy,
+		expanded_key,
+		&secp_ctx,
+	)
+	.unwrap();
+	// Ensure that dummy hops are added to the blinded path.
+	assert_eq!(blinded_path.blinded_hops().len(), 6);
+	let destination = Destination::BlindedPath(blinded_path);
+	let instructions = MessageSendInstructions::WithoutReplyPath { destination };
+	nodes[0].messenger.send_onion_message(test_msg, instructions).unwrap();
+	nodes[1].custom_message_handler.expect_message(TestCustomMessage::Pong);
+	pass_along_path(&nodes);
+}
+
 #[test]
 fn two_unblinded_two_blinded() {
 	let nodes = create_nodes(5);
@@ -648,8 +676,9 @@ fn test_blinded_path_padding_for_full_length_path() {
 	// Update the context to create a larger final receive TLVs, ensuring that
 	// the hop sizes vary before padding.
 	let context = MessageContext::Custom(vec![0u8; 42]);
-	let blinded_path = BlindedMessagePath::new(
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
 		&intermediate_nodes,
+		5,
 		nodes[3].node_id,
 		context,
 		&*nodes[3].entropy_source,
@@ -683,8 +712,9 @@ fn test_blinded_path_no_padding_for_compact_path() {
 	// Update the context to create a larger final receive TLVs, ensuring that
 	// the hop sizes vary before padding.
 	let context = MessageContext::Custom(vec![0u8; 42]);
-	let blinded_path = BlindedMessagePath::new(
+	let blinded_path = BlindedMessagePath::new_with_dummy_hops(
 		&intermediate_nodes,
+		5,
 		nodes[3].node_id,
 		context,
 		&*nodes[3].entropy_source,