- Move the metadata and MusicBrainz-related tests into a dedicated tests/metadata subfolder.
- Keep the rest of the suite flat for now.
- Preserve the existing test filenames so the change stays organizational rather than behavioral.
Covers original ordering preservation, partial/full hit thresholds,
empty result_ids, TTL expiration, cache miss behavior, and a
round-trip count assertion confirming 50 entities resolve in a
single SELECT (not 50).