chanmon_consistency: tolerate drained splice tx in SplicePending handler

During settlement, the drain loop removes all transactions from
broadcasters unconditionally, but confirm_tx may reject a splice
transaction if its inputs are already spent. When a SplicePending
event then fires, the tx is neither in the broadcaster nor in
confirmed_txids, causing a panic.

Replace the .expect() with an if-let to gracefully skip splice
transactions that were already drained.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -2043,12 +2043,16 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(
 									_ => &broadcast_c,
 								};
 								let mut txs = broadcaster.txn_broadcasted.borrow_mut();
-								let pos = txs
+								if let Some(pos) = txs
 									.iter()
 									.position(|tx| new_funding_txo.txid == tx.compute_txid())
-									.expect("SplicePending but splice tx not found in broadcaster");
-								let splice_tx = txs.remove(pos);
-								chain_state.confirm_tx(splice_tx);
+								{
+									let splice_tx = txs.remove(pos);
+									chain_state.confirm_tx(splice_tx);
+								}
+								// If not found, the settlement drain loop already
+								// removed it from the broadcaster but confirm_tx
+								// rejected it (e.g. inputs already spent).
 							}
 						},
 						events::Event::SpliceFailed { .. } => {},