gateway: rewrite transcripts atomically to avoid mid-write corruption

[Xowiek]

Summary

SessionStore.rewrite_transcript() was rewriting legacy JSONL transcripts in-place with "w" mode. If Hermes crashed or a serialization error happened mid-rewrite, the transcript file could be left truncated even though the previous version was still valid.

This patch makes transcript rewrites atomic by writing to a temporary sibling file, flushing and fsyncing it, and then swapping it into place with os.replace().

Why

This is a small reliability fix, but it closes a real failure mode in a sensitive code path used by /retry, /undo, and /compress.

Before this change:

• a mid-write failure could partially overwrite the JSONL transcript
• legacy sessions that still rely on JSONL fallback could lose usable history
• subsequent load_transcript() calls could observe truncated state

After this change:

• the old transcript remains intact unless the replacement file is fully written
• interruptions and serialization failures no longer corrupt the existing JSONL transcript
• rewrite behavior is consistent with the repo’s other atomic-write patterns

Changes

• update gateway/session.py so rewrite_transcript() uses:
  • temp file in the same directory
  • flush() + os.fsync()
  • atomic os.replace()
  • temp-file cleanup on failure
• add regression coverage in tests/gateway/test_session.py for a simulated mid-write serialization failure

Test coverage

Ran:

uv run --extra dev pytest tests\gateway\test_session.py -k "test_rewrite_failure_preserves_existing_transcript" -v

Result:
PASSED

Also verified the broader test_session.py gateway session tests completed successfully.

[teknium1]

Thanks for the contribution, @Xowiek!

Closing this as a duplicate of #4985 (by @binhnt92), which targets the same fix/feature. We're consolidating on that PR for review.

If you want to help push it over the line, please jump in there — or if you think your approach is better for a specific reason that isn't covered in the other PR, let us know and we can reopen.