platform: use FILE_FLAG_WRITE_THROUGH on Windows for SyncFile data durability, fixes #9388

[mr-raj12]

Description

Implement Windows-native SyncFile using CreateFileW with FILE_FLAG_WRITE_THROUGH to bypass volatile write caches and write directly to persistent storage. This is the Windows equivalent of the macOS F_FULLFSYNC fix (#9383 / PR #9385).

Fixes #9388

Changes

• src/borg/platform/windows.pyx — add SyncFile subclass using CreateFileW + FILE_FLAG_WRITE_THROUGH; falls back to base when an existing fd is provided (e.g. SaveFile)
• src/borg/platform/__init__.py — import SyncFile from .windows instead of .base
• src/borg/platform/base.py — update resolved TODO comment
• src/borg/testsuite/platform/platform_test.py — add skipif_not_win32 marker
• src/borg/testsuite/platform/windows_test.py — 6 tests (write/read, FileExistsError, text mode, fd fallback, sync, write-through flag verification)

Checklist

• PR is against master (or maintenance branch if only applicable there)
• New code has tests and docs where appropriate
• Tests pass (run tox or the relevant test subset)
• Commit messages are clean and reference related issues

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 76.43%. Comparing base (0354697) to head (59dcf0e).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/borg/platform/__init__.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #9389   +/-   ##
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  Coverage   76.43%   76.43%           
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  Files          85       85           
  Lines       14800    14800           
  Branches     2212     2212           
=======================================
  Hits        11313    11313           
  Misses       2809     2809           
  Partials      678      678           

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