cache: add seek()/tell() to SyncFile, use SaveFile in _write_files_cache, fixes #9390

[mr-raj12]

Description

• _write_files_cache() in cache.py has a TODO noting that SaveFile couldn't be used because SyncFile lacks .seek() and .tell()
• Without SaveFile, the files cache is written directly to the final path — a crash mid-write can leave a corrupted file
• SaveFile provides atomic writes (temp file + os.replace), but its __enter__ returns a SyncFile, and IntegrityCheckedFile needs .seek()/.tell() on its backing fd for hash finalization (hash_length calls seek(0, SEEK_END) + tell())
• Fix: add the missing methods to SyncFile, make close() idempotent (needed because the IntegrityCheckedFile → hasher → FileLikeWrapper exit chain closes the SyncFile before SaveFile.__exit__ tries to close it again), then refactor _write_files_cache to use SaveFile

Fixes #9390

Changes

File	Change
src/borg/platform/base.py	Add seek() and tell() to SyncFile, delegating to self.f; make close() idempotent with if self.f.closed: return guard
src/borg/cache.py	Refactor _write_files_cache to wrap writes in SaveFile + IntegrityCheckedFile(override_fd=...) for atomic updates; remove TODO comment
src/borg/testsuite/platform/all_test.py	Add tests for seek/tell, idempotent close, and SaveFile + IntegrityCheckedFile integration
src/borg/testsuite/crypto/file_integrity_test.py	Add test for IntegrityCheckedFile with SyncFile as override_fd

Checklist

• PR is against master
• New code has tests
• Tests pass
• Commit messages are clean and reference related issues
• Code formatted with black

[codecov]

Codecov Report

❌ Patch coverage is 89.65517% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.46%. Comparing base (0354697) to head (6c1017b).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/cache.py	86.36%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9392      +/-   ##
==========================================
+ Coverage   76.43%   76.46%   +0.02%     
==========================================
  Files          85       85              
  Lines       14800    14810      +10     
  Branches     2212     2213       +1     
==========================================
+ Hits        11313    11324      +11     
  Misses       2809     2809              
+ Partials      678      677       -1     

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[ThomasWaldmann]

good change, found some minor things.