fix(gateway): rewrite transcripts atomically

[binhnt92]

rewrite_transcript() still overwrites the legacy JSONL transcript with a bare
open(path, "w"). If the gateway dies mid-write during /retry, /undo, or
/compress, the transcript file can be left empty or partially rewritten.

This refresh trims the PR down to the still-reproducible bug on current main.
channel_directory already uses atomic_json_write(), so it is no longer part
of the fix.

Changes Made

• Rewrite transcripts through a temp file in the same directory
• fsync() the temp file before replacing the original transcript
• Remove the temp file on failure so aborted rewrites do not leave garbage behind
• Add a regression test proving a mid-rewrite failure leaves the old transcript intact

How to Test

python3 -m pytest tests/gateway/test_session.py -k rewrite -v
python3 -m pytest tests/gateway/test_session.py -v

Checklist

• Tests added
• Existing session test file passes
• Tested on Linux (Ubuntu 22.04)

[binhnt92]

channel_directory looks fixed on main already. rewrite_transcript() still reproduces, so I'd keep this open but trim it to that part.

[alt-glitch]

Likely duplicate of #15085 and #8065 — same atomic rewrite_transcript() fix for JSONL corruption on crash. Multiple competing PRs exist for this issue.

[alt-glitch]

Likely duplicate of #15085 and #8065 — same atomic rewrite_transcript() fix for JSONL corruption on crash.

[binhnt92]

Closing this to reduce duplicate PR noise. There are multiple competing PRs for the same atomic rewrite_transcript() issue (#8065, #8077, #15085, #20012), and a collaborator already flagged this one as likely duplicate.