Skip ChannelManager persistence for message-only monitor completions

When process_pending_monitor_events processes only Completed events
and the resulting work is limited to message-only monitor
completion handling, ChannelManager persistence can be skipped.

Completion handling now reports whether it actually mutated
ChannelManager state, and process_pending_monitor_events uses
that to decide between SkipPersistHandleEvents and DoPersist.

AI tools were used in preparing this commit.

Author: joostjager