Settings now cross the wire through ServerSettingsWire: compactIdleThreads
stays the boolean older clients decode, and the mode rides beside it in
idleCompactionMode. Clients read servers that send only the boolean or only
the mode, and settings patches accept the legacy boolean. Persisted settings
keep the mode string.

Fixes #23109.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>