fuzz: cover deferred writing in chanmon_consistency #22965

Job	Run time
ext-test	8m 8s
fuzz_sanity	6m 52s
tor-connect	5m 40s
check_docs	5m 10s
rustfmt	14s
benchmark	17m 49s
build-cfg-flags / build (self-hosted, 1.75.0)	11m 28s
check_release	3m 10s
build-features / build (self-hosted, 1.75.0)	35m 9s
fuzz	51m 31s
build-bindings / build (self-hosted, 1.75.0)	18m 45s
build-sync / build (self-hosted, 1.75.0)	20m 37s
linting	2m 28s
build-workspace / build (self-hosted, 1.75.0)	13m 19s
build-nostd / build (self-hosted, 1.75.0)	21m 38s
coverage	27m 28s
notify-failure	0s
	4h 9m 26s

Provide feedback