fix: honor Postgres search path in table checks

[he-yufeng]

Summary

• make PostgreSQL table existence checks follow the active search_path with to_regclass
• reuse that helper when creating the full entity/relation tables instead of checking only public
• add focused coverage for the search-path-aware table check

To verify

• python -m pytest tests\kg\postgres_impl\test_postgres_search_path_table_exists.py -q
• python -m pytest tests\kg\postgres_impl\test_postgres_client_manager.py tests\kg\postgres_impl\test_postgres_migration.py -q
• python -m py_compile lightrag\kg\postgres_impl.py tests\kg\postgres_impl\test_postgres_search_path_table_exists.py
• git diff --check

[danielaskdd]

Thanks for sharing.