scrub: don't abort import on illegal MP4 album art

[MwC-Trexx]

Bug Description

Fixes #2498

scrub extracts embedded album art, strips tags, then writes the art back. For MP4/M4A files whose cover is not JPEG or PNG, mediafile can read the image but raises ValueError: MP4 files only supports PNG and JPEG images on restore. That exception was uncaught, so a single bad cover aborted the entire import.

Reproduced on beets 2.14.1 during a quiet bulk import of a large library (same traceback as #2498 / #2713).

Root Cause

beetsplug/scrub.py _scrub_item only caught mediafile.UnreadableFileError around mf.images = images. mediafile.storage.mp4.MP4ImageStorageStyle.serialize raises ValueError for any other MIME type.

Fix

Catch ValueError, OSError, and mutagen.MutagenError on art restore, log the path, and continue without art. Tags are still rewritten.

How to Verify

1. Embed a TIFF (or GIF) as an MP4 covr atom.
2. Run beet import / beet scrub with scrub.auto enabled.
3. Import should complete; the file keeps database-tracked tags and drops the illegal art.

Test Plan

• Added regression test test_illegal_mp4_art_does_not_crash_restore (embeds TIFF into min.m4a, asserts _scrub_item does not raise)
• Confirmed the new test fails on unpatched scrub.py with the original ValueError
• pytest test/plugins/test_scrub.py — 4 passed
• Changelog + scrub plugin docs updated

Risk Assessment

Low — only the art-restore failure path changes; successful JPEG/PNG restore is unchanged. Worst case for illegal art is the previous implicit outcome (no art) without taking down the import.

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.41%. Comparing base (5e6d9ad) to head (4ab8b07).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/scrub.py	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7047      +/-   ##
==========================================
- Coverage   77.42%   77.41%   -0.02%     
==========================================
  Files         163      163              
  Lines       21870    21882      +12     
  Branches     3374     3377       +3     
==========================================
+ Hits        16933    16939       +6     
- Misses       4120     4124       +4     
- Partials      817      819       +2     

Files with missing lines	Coverage Δ
beetsplug/scrub.py	64.70% <83.33%> (+3.06%)	⬆️

... and 2 files with indirect coverage changes

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[semohr]

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A few thoughts:

• I’m hesitant to broadly catch ValueError, OSError, and MutagenError, as this could hide unrelated issues.
• Ideally, this should be fixed directly in Mutagen. If there isn’t an issue yet, I’d suggest opening one :)
• If an upstream fix is likely to take some time, we could add a specific error in mediafile for unsupported artwork and handle that explicitly in scrub. Since we maintain mediafile, moving the fix there shouldn’t be a problem.