Fix Kuzu shared-connection race with query lock - #881

Summary

• serialize all Kuzu conn.execute() calls behind a shared hreading.RLock
• thread _query_lock through KuzuDriverWrapper and KuzuSessionWrapper
• add debug logging for idempotent �lready exists collisions
• document cooperative-yield safety boundary in indexing pipeline
• add dedicated concurrency regression tests for lock plumbing, serialization, reentrancy, and error behavior

Validation

• python -m pytest tests/unit/core/test_kuzu_connection_lock.py -q -> 12 passed
• python -m pytest tests/unit/core/test_database.py -q -> 15 passed
• full unit suite has existing unrelated baseline failures in this environment

Fixes #880

@Shashankss1205 , could you please take a look when you have time?

Happy to incorporate any feedback including additional tests, refinements or scope adjustments if needed.