fix: tolerate missing thumbnails in all parsers

[sigma67]

Closes #977

#974 made parse_playlist tolerant of entries with no thumbnail renderer, but every other parser still read thumbnail data unconditionally, so the same kind of entry ({} where a renderer is expected) aborts the whole listing elsewhere too.

This applies none_if_absent=True to all remaining thumbnail lookups:

• parsers/browsing.py: parse_album, parse_single, parse_song, parse_song_flat, parse_related_artist, parse_watch_playlist
• parsers/albums.py: parse_album_header, parse_album_header_2024
• parsers/playlists.py: parse_playlist_header_meta (the THUMBNAIL_CROPPED fallback in parse_playlist_header already handles None)
• parsers/explore.py: parse_chart_playlist, parse_chart_artist
• parsers/library.py: parse_albums
• parsers/podcasts.py: parse_base_header, parse_episode, parse_episode_flat, parse_podcast
• parsers/watch.py: parse_watch_track
• mixins/podcasts.py: get_channel

parsers/uploads.py was already guarded by an if "thumbnail" in data check, now expressed the same way as the rest.

THUMBNAIL_OVERLAY* lookups in parsers/search.py are untouched — those read videoId/videoType, not thumbnail data.

Tests

tests/parsers/test_thumbnails.py covers the parsers that take a plain renderer dict, asserting thumbnails is None instead of a KeyError. All 11 fail on main and pass with this change. parse_song_flat, parse_episode/parse_episode_flat and parse_uploaded_items need substantially larger fixtures and are covered only by the existing live-API tests.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.85%. Comparing base (8d43d07) to head (369e86f).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #978      +/-   ##
==========================================
+ Coverage   95.78%   95.85%   +0.07%     
==========================================
  Files          47       47              
  Lines        2729     2729              
==========================================
+ Hits         2614     2616       +2     
+ Misses        115      113       -2     

Flag	Coverage Δ
unittests	95.85% <100.00%> (+0.07%)	⬆️

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