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

@@ -418,6 +418,23 @@ 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 blinded_path =
+		BlindedMessagePath::new_with_dummy_hops(&[], 5, nodes[1].node_id, context, entropy, &secp_ctx).unwrap();
+	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);