chore: improve beets maintenance path

[lphuc2250gma]

Summary:

• Add or tighten focused edge-case tests or type assertions in test/test_util.py, test/test_files.py related to Python typing, tests, CLI ergonomics, observability; avoid docs-only changes and broad refactors.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.

[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 75.58%. Comparing base (8152e43) to head (ca71da5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6712   +/-   ##
=======================================
  Coverage   75.57%   75.58%           
=======================================
  Files         162      162           
  Lines       20810    20810           
  Branches     3294     3294           
=======================================
+ Hits        15727    15729    +2     
+ Misses       4307     4306    -1     
+ Partials      776      775    -1     

see 1 file with indirect coverage changes

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[snejus]

Would you mind refactoring these tests to use pytest.mark.parametrize?

[snejus]

What is this one for?