lastgenre: Album stage fall back to multi-valued albumartists

[JOJ0]

Description

Fixes: #6238

If last.fm returns no album genres for albumartist, fall back to querying the album against each artist in albumartists (falling through to the artist stage if that also yields nothing.)

The album stage reasoning is now slightly more complex and was moved to its own helper method.

To Do

• Documentation.
• Changelog.
• 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

❌ Patch coverage is 72.22222% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.44%. Comparing base (ad758c1) to head (aa9a80a).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/lastgenre/__init__.py	72.22%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6893   +/-   ##
=======================================
  Coverage   77.44%   77.44%           
=======================================
  Files         163      163           
  Lines       21945    21957   +12     
  Branches     3386     3389    +3     
=======================================
+ Hits        16995    17005   +10     
- Misses       4128     4129    +1     
- Partials      822      823    +1     

Files with missing lines	Coverage Δ
beetsplug/lastgenre/__init__.py	87.04% <72.22%> (-0.16%)	⬇️

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[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.

[Copilot]

Pull request overview

PR make lastgenre album stage smarter for multi-artist albums. When Last.fm give no album tags for main albumartist, plugin now try album lookup for each value in multi-valued albumartists, then still fall through to artist stage if nothing found.

Changes:

• Add _try_resolve_album_stage helper to hold album-stage logic and multi-valued albumartists fallback.
• Add focused unit test that exercises per-albumartist album lookup fallback.
• Document new multi-artist album behavior in lastgenre docs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
beetsplug/lastgenre/__init__.py	Add album-stage helper that falls back to per-albumartists album lookups when primary album lookup empty.
test/plugins/test_lastgenre.py	Add direct test for _try_resolve_album_stage fallback behavior.
docs/plugins/lastgenre.rst	Document album-stage fallback to multi-valued albumartists before artist source.