fix(session): atomic write for sessions.json to prevent data loss on crash

[alireza78a]

What

_save() in gateway/session.py was writing sessions.json like this:

with open(sessions_file, "w") as f:
    json.dump(data, f, indent=2)

open(..., "w") truncates the file immediately. If the process crashes or gets OOM-killed mid-write, sessions.json ends up empty. On next startup _ensure_loaded() silently skips the broken file — every user gets a fresh session.

Fix

Replaced with tempfile.mkstemp + os.replace — same pattern already used in cron/jobs.save_jobs():

1. Write to a temp file in the same directory
2. fsync to flush to disk
3. os.replace — atomic rename, readers always see a complete file

Impact

• Prevents silent session loss on crash or OOM kill
• No behaviour change under normal operation

[alireza78a]

Checked the failing test — it fails on main too, unrelated to this change.

[teknium1]

Merged in 8f0b07e. Good catch @alireza78a — the truncate-on-crash risk was real. Resolved a minor conflict (added encoding='utf-8' to the fdopen call from a parallel change). Matches our existing atomic write pattern in cron/jobs.py.