Fix filtered MusicBrainz track totals

[iamdhrv]

Change

• derive each MusicBrainz track's medium_total from the tracks remaining after
  data/video filtering
• extend the existing data/video matrix to assert the reported medium total for
  every configuration

Why

Video tracks live in MusicBrainz's normal track list, so filtering them left
medium_total at the original unfiltered count. Data tracks are represented
separately, which produced inconsistent metadata for the two equivalent ignore
options. Reporting the filtered count keeps the candidate's per-medium metadata
consistent with the tracks Beets actually imports.

Checks

• uv run ruff check beetsplug/musicbrainz.py test/plugins/test_musicbrainz.py
• uv run pytest -q test/plugins/test_musicbrainz.py (55 passed)

Fixes #6836

[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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.62%. Comparing base (12e261c) to head (d408476).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6887   +/-   ##
=======================================
  Coverage   75.62%   75.62%           
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  Files         163      163           
  Lines       21318    21318           
  Branches     3361     3361           
=======================================
  Hits        16122    16122           
  Misses       4405     4405           
  Partials      791      791           

Files with missing lines	Coverage Δ
beetsplug/musicbrainz.py	95.00% <100.00%> (ø)

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[snejus]

Thanks!