Add test for refund creation with insufficient liquidity

Adds the `fails_creating_refund_with_insufficient_liquidity` test to
verify that the `create_refund_builder` method correctly handles
cases where there is insufficient channel liquidity. The test verifies
that the method returns the `Bolt12RequestError::InsufficientLiquidity`
error when the refund amount exceeds the available liquidity.

Author: slanesuke

lightning/src/ln/offers_tests.rs

@@ -2369,4 +2369,26 @@ fn fails_paying_offer_with_insufficient_liquidity() {
 			assert_eq!(e, Bolt12RequestError::InsufficientLiquidity);
 		}
 	}
+}
+
+#[test]
+fn fails_creating_refund_with_insufficient_liquidity() {
+	let chanmon_cfgs = create_chanmon_cfgs(2);
+	let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
+	let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+	let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
+
+	create_announced_chan_between_nodes_with_value(&nodes, 0, 1, 10_000_000, 1_000_000_000);
+
+	let bob = &nodes[1];
+
+	let absolute_expiry = Duration::from_secs(u64::MAX);
+	let payment_id = PaymentId([1; 32]);
+	let refund = bob.node
+		.create_refund_builder(950_000_000, absolute_expiry, payment_id, Retry::Attempts(0), None);
+
+	match refund {
+		Ok(_) => panic!("Expected error with insufficient liquidity."),
+		Err(e) => assert_eq!(e, Bolt12RequestError::InsufficientLiquidity),
+	}
 }