json.load reads a huge integer as an int, and math.isfinite raised
OverflowError on it outside any try: conversation_in_use and purge_artifacts
(which must never raise) threw, where 6f4f40a had taken it as live. The
conversion to float now comes first, and an overflow means live.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>