convert: skip missing album art instead of crashing

[cycsmail]

Description

Fixes #4692.

convert -a with copy_album_art enabled crashes with a FileNotFoundError when the album's stored art path points at a file that no longer exists on disk (for example a multi-disc album whose cover lives in the album root rather than in each per-disc directory). This adds an os.path.isfile guard in copy_album_art so a missing source is logged and skipped instead of aborting the conversion, using the same util.syspath convention already used for the destination check just below.

To Do

• Documentation. (No new flags or behaviour to document.)
• Changelog.
• Tests. (Added test_copy_album_art_missing_source, which reproduces the crash without this change and passes with it.)

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.57%. Comparing base (cb564a6) to head (4017906).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beetsplug/convert.py	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6749      +/-   ##
==========================================
+ Coverage   74.52%   74.57%   +0.05%     
==========================================
  Files         162      162              
  Lines       20818    20821       +3     
  Branches     3295     3296       +1     
==========================================
+ Hits        15515    15528      +13     
+ Misses       4547     4536      -11     
- Partials      756      757       +1     

Files with missing lines	Coverage Δ
beetsplug/convert.py	77.05% <66.66%> (+2.28%)	⬆️

... and 1 file with indirect coverage changes

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