chanmon_consistency: accept ChannelUnavailable on splice after force-close

The splice_channel error handler only expected APIMisuseError with a
splice-related message. When a channel has been force-closed, the splice
call returns ChannelUnavailable instead, which caused the assertion to
panic. Allow ChannelUnavailable as a valid error variant.

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

Author: joostjager

fuzz/src/chanmon_consistency.rs

@@ -1455,7 +1455,8 @@ pub fn do_test<Out: Output + MaybeSend + MaybeSync>(
 			},
 			Err(e) => {
 				assert!(
-					matches!(e, APIError::APIMisuseError { ref err } if err.contains("splice")),
+					matches!(e, APIError::APIMisuseError { ref err } if err.contains("splice"))
+					|| matches!(e, APIError::ChannelUnavailable { .. }),
 					"{:?}",
 					e
 				);