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

• 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

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.84%. Comparing base (59a456e) to head (4417fea).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6783   +/-   ##
=======================================
  Coverage   74.84%   74.84%           
=======================================
  Files         163      163           
  Lines       20949    20949           
  Branches     3300     3300           
=======================================
  Hits        15680    15680           
  Misses       4511     4511           
  Partials      758      758           

Files with missing lines	Coverage Δ
beets/dbcore/db.py	94.40% <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

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