#466 Race Condition: _last_cache_check_time modified without lock in http_request.py

engine/tools/http_request.py:209-231 modifies module-level _last_cache_check_time without synchronization. Multiple concurrent requests can interleave reads/writes to this timestamp, causing: 1. Multiple redundant cache checks 2. Inconsistent timestamp state FIX: Use threading.Lock to protect the check-and-update operation.