Fix audit warnings about empty string defaults hiding data corruption:
1. get_connection_info(): Validate required fields (user_id, connected_at,
last_seen) exist before creating ConnectionInfo. Return None and log
warning for corrupted records instead of returning invalid data.
2. unregister_connection(): Log warning if user_id is missing during
cleanup. Existing code safely handles this (skips cleanup), but now
we have visibility into data corruption.
Test added for corrupted data case.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
0a9cd73daf