Added import threading to src/cleveragents/providers/registry.py to enable a module-level lock.
Defined _registry_lock: threading.Lock = threading.Lock() at module scope.
Wrapped get_provider_registry() body with with _registry_lock: to ensure thread-safety.
Wrapped reset_provider_registry() body with with _registry_lock: to ensure thread-safety.
Added TDD support: features/tdd_registry_thread_safety.feature with @tdd_issue @tdd_issue_10409 @mock_only.
Added step definitions: features/steps/tdd_registry_thread_safety_steps.py.
ISSUES CLOSED: #10478