fix: match playlist queries with relative path storage

[cookesan]

Summary

• normalize playlist query entries to the stored path form before the SQL IN lookup
• add coverage for playlists whose entries point inside the active library directory
• add the unreleased changelog entry for The playlist plugin is not returning any results #6784

Fixes #6784.

Tests

• .venv/bin/python -m pytest test/plugins/test_playlist.py -q
• .venv/bin/python -m pytest test/plugins/test_playlist.py test/test_query.py -q
• .venv/bin/python -m pytest test/test_library.py test/library/test_migrations.py -q
• .venv/bin/ruff format --check --diff beetsplug/playlist.py test/plugins/test_playlist.py
• .venv/bin/ruff check beetsplug/playlist.py test/plugins/test_playlist.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.84%. Comparing base (37e2a8b) to head (a49c785).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6792      +/-   ##
==========================================
- Coverage   74.85%   74.84%   -0.01%     
==========================================
  Files         163      163              
  Lines       20947    20948       +1     
  Branches     3299     3299              
==========================================
- Hits        15680    15679       -1     
- Misses       4510     4511       +1     
- Partials      757      758       +1     

Files with missing lines	Coverage Δ
beetsplug/playlist.py	87.27% <100.00%> (+0.11%)	⬆️

... and 1 file with indirect coverage changes

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[snejus]

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