Fix update with plugin media flex fields

[gaoflow]

Summary

• avoid treating flexible attributes passed to Model.store(fields=...) as fixed table columns
• add a dbcore regression test for storing a flexattr included in fields
• add an update command regression test for plugin-added media fields
• add a changelog note for Update mediafields regression #5580

Fixes #5580.

Tests

• poe test test/test_dbcore.py test/ui/commands/test_update.py
• poe lint
• poe check-format

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.63%. Comparing base (090ed24) to head (cdd48a2).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6783   +/-   ##
=======================================
  Coverage   75.63%   75.63%           
=======================================
  Files         163      163           
  Lines       21318    21318           
  Branches     3361     3361           
=======================================
  Hits        16124    16124           
  Misses       4404     4404           
  Partials      790      790           

Files with missing lines	Coverage Δ
beets/dbcore/db.py	94.41% <100.00%> (ø)

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[semohr]

Tests

• uv run --with pytest --with mediafile --with confuse --with jellyfish --with munkres --with musicbrainzngs --with unidecode --with typing_extensions --with-editable . pytest test/test_dbcore.py test/ui/commands/test_update.py -q
• uv run --with ruff ruff check --config=pyproject.toml beets/dbcore/db.py test/test_dbcore.py test/ui/commands/test_update.py
• uv run --with ruff ruff format --check --config=pyproject.toml beets/dbcore/db.py test/test_dbcore.py test/ui/commands/test_update.py
• git diff --check

Please read our contribution guide! We are not using uv in our dev setup currently.

[gaoflow]

Sorry — that was my own scratch invocation, not your dev setup. Swapped the description over to the poe tasks. I'll rebase this too, it's picked up a conflict.

[snejus]

Lovely, thanks for this. Note CI will fail because you will need to move your changelog note under Unreleased section since I've just released a new version of beets.