6 tests pin the Plex client surface the engine will dispatch
through after Phase B/C migrations:
- is_connected returns False on no-server, True on server-present
- is_fully_configured requires BOTH server AND music_library
- get_all_artists empty list on not-connected, iterates
music_library.searchArtists() when connected
- get_all_album_ids returns a set of STRING ratingKey values
(coerced from Plex ints so semantics match Jellyfin GUIDs +
Navidrome string ids)
Phase A pinning catches behavior drift during web_server.py
dispatch-site migrations (Phase C) and engine adapter wiring
(Phase B).
