Use va_name config for all artist fields on VA releases

[wavebyrd]

Summary

Fixes #6316

When importing compilations/various artists albums, several fields used the
hardcoded string "Various Artists" instead of the user-configured va_name
setting:

• In the musicbrainz plugin, only info.artist was overridden with va_name
  when a release was identified as VA. The artist_sort, artists_sort,
  artist_credit, artists_credit, and artists fields were left with the raw
  MusicBrainz value ("Various Artists"), which then propagated to
  albumartist_sort, albumartists_sort, albumartist_credit,
  albumartists_credit, and albumartists on items.

• In the beatport plugin, the VA artist name was hardcoded to
  "Various Artists" instead of reading from config.

Changes

• beetsplug/musicbrainz.py: When info.va is true, override all artist-related
  fields (artist_sort, artists, artists_sort, artist_credit,
  artists_credit) with va_name, not just artist.
• beetsplug/beatport.py: Replace hardcoded "Various Artists" with
  config["va_name"].as_str().
• docs/changelog.rst: Add changelog entries for both fixes.

Test plan

• Import a VA album from MusicBrainz with a custom va_name setting (e.g.,
  va_name: "Artistes divers") and verify that albumartist_sort,
  albumartists_sort, albumartist_credit, albumartists_credit, and
  albumartists all use the configured value.
• Run beet mbsync on an existing VA album after changing va_name and verify
  the sort/credit fields update correctly.
• Verify that without a custom va_name, the default "Various Artists" is still
  used everywhere (no behavior change for default config).

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 69.55%. Comparing base (f203bc5) to head (be722b5).
⚠️ Report is 2 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/beatport.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6438   +/-   ##
=======================================
  Coverage   69.55%   69.55%           
=======================================
  Files         141      141           
  Lines       18493    18494    +1     
  Branches     3025     3025           
=======================================
+ Hits        12862    12863    +1     
  Misses       4995     4995           
  Partials      636      636           

Files with missing lines	Coverage Δ
beetsplug/beatport.py	43.69% <50.00%> (+0.23%)	⬆️

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[snejus]

Thanks!