fix(autotag): only write mb_* fields when data source is MusicBrainz

[aaronk6]

Description

Plugins like Spotify and Deezer set album_id, artist_id, and track_id on their AlbumInfo/TrackInfo objects, which apply_metadata() and apply_item_metadata() then unconditionally write to mb_albumid, mb_artistid, etc. This means service-specific IDs end up stored in MusicBrainz fields, breaking clients such as Music Assistant.

This PR adds a guard so that mb_* fields are only written when data_source == "MusicBrainz". Let me know if this is a good fix. If so, I’ll update the changelog and add some unit tests.

Note that a broader approach (registering Spotify/Deezer IDs as their own media fields) was explored in #6349, but it turned out to be quite complex, so this PR is just fixing the obvious problem.

To Do

• Documentation.
• Changelog.
• Tests.

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[Copilot]

Pull request overview

PR make sure beets only write mb_* id fields when match data come from MusicBrainz, so Spotify/Deezer/etc ids no longer get stuffed into MusicBrainz slots and break downstream clients.

Changes:

• Guard apply_item_metadata() so it only writes mb_trackid / mb_artistid(s) when track_info.data_source == "MusicBrainz".
• Guard apply_metadata() so it only writes all mb_* ids when album_info.data_source == "MusicBrainz".

[Copilot]

grug want test. new behavior: when AlbumInfo/TrackInfo data_source not MusicBrainz (Spotify, Deezer), mb_* fields should not get service id. add regression test in test/autotag/test_autotag.py for apply_metadata + apply_item_metadata to prove mb_* stay empty/cleared.

[aaronk6]

I’ll add the test once I have maintainer feedback.

[snejus]

This will break beets for all items that are not from MusicBrainz. As I mentioned previously, these fields act as central ID storage mechanism for any data source. For example, see data source counts in my personal library:

7       youtube
8       spotify
12      Deezer
16      Spotify
201     soundcloud
395     MusicBrainz
1147    Discogs
7586    bandcamp