fuzz: handle BroadcastChannelUpdate in chanmon

The regression was introduced in d627ce1. That change
switched fee update opcodes in chanmon_consistency from
maybe_update_chan_fees() to timer_tick_occurred(), which can
enqueue BroadcastChannelUpdate events while peers are
disconnected.

The harness already tolerated those events in one delivery
path, but still treated them as unreachable in
push_excess_b_events and disconnect draining. Accept
BroadcastChannelUpdate in those match arms so the fuzz target
no longer panics on valid timer tick driven message queues.

AI tools were used in preparing this commit.

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -1608,6 +1608,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 						},
 						MessageSendEvent::SendChannelReady { .. } => continue,
 						MessageSendEvent::SendAnnouncementSignatures { .. } => continue,
+						MessageSendEvent::BroadcastChannelUpdate { .. } => continue,
 						MessageSendEvent::SendChannelUpdate { ref node_id, .. } => {
 							if Some(*node_id) == expect_drop_id { panic!("peer_disconnected should drop msgs bound for the disconnected peer"); }
 							*node_id == a_id
@@ -1894,6 +1895,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 							MessageSendEvent::SendStfu { .. } => {},
 							MessageSendEvent::SendChannelReady { .. } => {},
 							MessageSendEvent::SendAnnouncementSignatures { .. } => {},
+							MessageSendEvent::BroadcastChannelUpdate { .. } => {},
 							MessageSendEvent::SendChannelUpdate { .. } => {},
 							MessageSendEvent::HandleError { ref action, .. } => {
 								assert_action_timeout_awaiting_response(action);
@@ -1916,6 +1918,7 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(data: &[u8], out: Out) {
 							MessageSendEvent::SendStfu { .. } => {},
 							MessageSendEvent::SendChannelReady { .. } => {},
 							MessageSendEvent::SendAnnouncementSignatures { .. } => {},
+							MessageSendEvent::BroadcastChannelUpdate { .. } => {},
 							MessageSendEvent::SendChannelUpdate { .. } => {},
 							MessageSendEvent::HandleError { ref action, .. } => {
 								assert_action_timeout_awaiting_response(action);