* fix: use atomic write in APIKeyManager.save() to prevent data loss
Opening api_keys.json with 'w' truncates the file before writing, so a
crash, disk-full, or mid-write error leaves all stored provider API keys
corrupted. Switch to atomic write (temp file + fsync + os.replace) so
the original file is always intact on any failure.
Fixes#4591
* chore: trigger CI re-run
* chore: update PR description
* chore: fix how-to-test section for description check
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Co-authored-by: michaelxer <michaelxer@users.noreply.github.com>
Michael