ReleaseGroupForRelease asks /ws/2/release/<id>?inc=release-groups through
the registered MusicBrainz provider, so it shares that provider's client
and 1 req/s limiter with tag enrichment and respects its on/off switch.
ErrNotFound when switched off or MusicBrainz has no such release (an id
that is already a release group included); ErrTransient to retry.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>