Fix symlinked reimports deleting library files

[atirna]

Description

Reimporting a library file through a symlinked parent could treat the same file as a duplicate because its stored and imported paths used different spellings. Choosing Remove then deleted the original file.

Fixes #6930.

Why

The importer now recognizes an existing in-library file by physical identity when its exact stored path is not found. Reimport replacement and in-library moves use the same resolved containment check, so the file stays in the library while its metadata is refreshed.

Verification

• Before: uv run pytest test/test_importer.py::ReimportTest::test_reimported_album_through_symlink_preserves_file -q failed because the reimport created a second file.
• After: uv run pytest test/test_importer.py -k 'ReimportTest or TestImportDuplicateAlbum or TestImportDuplicateSingleton' -q passes with 32 passed and 2 skipped.

To Do

• Documentation. (No user-facing option or configuration change.)
• Changelog. (Bug fix only; no new user-facing behavior.)
• 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 78.37838% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.20%. Comparing base (ba4787f) to head (2d8f336).
⚠️ Report is 65 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
beets/importer/tasks.py	78.37%	2 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6993      +/-   ##
==========================================
+ Coverage   77.19%   77.20%   +0.01%     
==========================================
  Files         163      163              
  Lines       21789    21813      +24     
  Branches     3360     3366       +6     
==========================================
+ Hits        16820    16841      +21     
- Misses       4149     4151       +2     
- Partials      820      821       +1     

Files with missing lines	Coverage Δ
beets/importer/tasks.py	92.72% <78.37%> (-0.19%)	⬇️

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