lastgenre: Refactor _get_genre

[JOJ0]

Description

The monolithic _get_genre method was broken down into the following private instance methods:

• Core Helpers:
  • _try_resolve_stage: Handles the canonicalization and logging of genres for a specific stage.
  • _fallback_stage: Provides the configured fallback genre.
• Lookup Stages:
  • _handle_existing_genres: Manages the initial check for pre-existing genres and the cleanup_existing logic.
  • _get_track_genre_stage: Fetches and resolves track-level genres.
  • _get_album_genre_stage: Fetches and resolves album-level genres.
  • _get_artist_genre_stage: Fetches and resolves artist-level genres, including multi-valued album artists and "Various Artists" logic.
  • _get_va_genre_stage: specifically handles the plurality logic for "Various Artists" albums.
• Fallbacks:
  • _get_original_fallback: Handles the "keep_existing" logic that attempts to use/canonicalize originally present genres if no new ones are found.

To Do

• Documentation
• Changelog
• Tests (_get_genre was already well covered and the signature of the method was kept)

[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

❌ Patch coverage is 65.26316% with 33 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.99%. Comparing base (f4b9436) to head (89cf4e5).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/lastgenre/__init__.py	65.26%	22 Missing and 11 partials ⚠️

Additional details and impacted files

@@                  Coverage Diff                  @@
##           lastgenre_aliases    #6474      +/-   ##
=====================================================
+ Coverage              69.96%   69.99%   +0.03%     
=====================================================
  Files                    146      146              
  Lines                  18642    18661      +19     
  Branches                3052     3054       +2     
=====================================================
+ Hits                   13042    13061      +19     
- Misses                  4957     4958       +1     
+ Partials                 643      642       -1     

Files with missing lines	Coverage Δ
beetsplug/lastgenre/__init__.py	84.21% <65.26%> (+0.98%)	⬆️

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