fix(test): wait for monitor update before cooperative close in fee change test

Add a 2s delay after the post-fee-change payment to let LDK's internal monitor updates settle, and use the cooperative_close_by_ldk helper.

Author: febyeji

tests/common/scenarios/channel.rs

@@ -270,10 +270,12 @@ pub(crate) async fn cooperative_close_after_fee_change<E: ElectrumApi>(
 	node.bolt11_payment().send(&parsed, None).unwrap();
 	expect_event!(node, PaymentSuccessful);
 
+	// Allow time for payment state and monitor updates to fully settle
+	// before initiating shutdown.
+	tokio::time::sleep(Duration::from_secs(2)).await;
+
 	// Cooperative close
-	let ext_node_id = peer.get_node_id().await.unwrap();
-	node.close_channel(&user_channel_id, ext_node_id).unwrap();
-	expect_channel_closed(node, true).await;
+	cooperative_close_by_ldk(node, peer, bitcoind, electrs, &user_channel_id).await;
 }
 
 /// Open a channel, mine many blocks, then force close.