platform: use F_FULLFSYNC on macOS for SyncFile data durability, fixes #9383

[mr-raj12]

Backport of #9385 to 1.4-maint.

SyncFile.sync() calls platform.fdatasync(self.fd), which on macOS resolves to os.fsync(). fsync() only flushes to the drive's write cache as it does not guarantee data reaches persistent storage. A power loss could silently lose data
that SyncFile believed was safely persisted.

The fix was acknowledged with a TODO in src/borg/platform/base.py: TODO: Use F_FULLSYNC on macOS.

This backport implements the platform-specific override suggested in #9383: add fdatasync() and sync_dir() to platform/darwin.pyx using fcntl(fd, F_FULLFSYNC) with an os.fsync() fallback for network filesystems that do not support F_FULLFSYNC.

Changes

File	Change
src/borg/platform/darwin.pyx	Add fdatasync() using fcntl F_FULLFSYNC with os.fsync() fallback; add sync_dir() using the same
src/borg/platform/__init__.py	Import fdatasync, sync_dir from .darwin in the darwin block
src/borg/platform/base.py	Remove resolved TODO comment
src/borg/testsuite/platform.py	Add 4 tests: F_FULLFSYNC usage, fsync fallback, fdatasync integration, sync_dir integration
docs/changes.rst	Add changelog entry for 1.4.x

Checklist

• PR is against 1.4-maint (maintenance branch as change is only applicable here as master is already fixed via platform: use F_FULLSYNC on macOS for SyncFile data durability, fixes #9383 #9385)
• New code has tests and docs where appropriate
• Tests pass
• Commit message references related issue

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 81.94%. Comparing base (0eaa76f) to head (58e1c0f).
⚠️ Report is 1 commits behind head on 1.4-maint.

Files with missing lines	Patch %	Lines
src/borg/platform/__init__.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           1.4-maint    #9592      +/-   ##
=============================================
+ Coverage      81.92%   81.94%   +0.01%     
=============================================
  Files             38       38              
  Lines          11306    11307       +1     
  Branches        1781     1781              
=============================================
+ Hits            9263     9265       +2     
  Misses          1456     1456              
+ Partials         587      586       -1     

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