fix(keyval): make FilesystemStore._set_raw durable and leak-free

[tomasvotava]

Follow-up to #185.

FilesystemStore._set_raw is the atomic-write pattern that #158/#185 was explicitly told to mirror. The gemini-code-assist review on #185 identified durability/leak gaps in the FileQueue copy; the keyval sibling had the same two:

• Durability: no flush()/fsync() before os.replace — a crash right after the rename could leave an empty/truncated value file. → now flush() + os.fsync().
• Temp-file leak: delete=False with no cleanup — a write failure stranded the temp file in root_path. → now removed via Path(...).unlink(missing_ok=True) on error, then re-raise.

The third FileQueue gap (text encoding) does not apply — this writer is binary ("wb").

Out of scope: LocalFileCache's write path has separate, larger safety issues already tracked in #156 — deliberately not folded in here.

Tests: new test_set_cleans_up_temp_file_on_error + existing suite (19 passed). ruff / ruff-format / mypy-strict / import-linter all green.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.64%. Comparing base (3226d80) to head (96ac1a8).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #188      +/-   ##
==========================================
+ Coverage   87.61%   87.64%   +0.02%     
==========================================
  Files          57       57              
  Lines        2932     2938       +6     
==========================================
+ Hits         2569     2575       +6     
  Misses        363      363              

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[gemini-code-assist]

Code Review

This pull request enhances the durability of the filesystem key-value store by implementing flush and fsync during writes and ensuring temporary files are deleted upon failure. A new test case validates the cleanup logic. Feedback suggests using a finally block instead of except Exception to more robustly handle BaseException types, such as KeyboardInterrupt, preventing potential file leaks.