test: run mechanism tests to validate recent changes

[hdviettt]

Context

Recent changes across 10+ files: retrieval improvements, extraction prompt changes, bare exception handler fixes, race condition fix. Need to verify nothing is broken.

Tasks

• Run pytest eval/mechanism/ -v — all 125 tests should pass
• Verify syntax on all modified backend files

Files changed

• eval/simulation/longmemeval_harness.py — extraction prompt, QA prompt, retrieval logic
• backend/memory/background_agent.py — extraction prompt, atomic SQL for contradiction penalty
• backend/memory/manager.py, consolidation.py, scheduler.py — exception handler fixes
• backend/api/chat.py, memory.py, knowledge.py — exception handler fixes
• backend/agent/graph.py, backend/main.py — exception handler fixes