test: add knowledge_graph metadata coverage

[fatkobra]

What does this PR do?

Adds focused tests for metadata and connection lifecycle behavior in mempalace/knowledge_graph.py.

Covered:

• entity properties persistence
• triple confidence, source_closet, and source_file persistence
• query_entity() exposure of confidence and source_closet
• close() + lazy reconnect behavior
• stats()["relationship_types"]

How to test

Validation:

• ruff check tests/test_knowledge_graph_metadata.py
• python -m pytest tests/test_knowledge_graph_metadata.py -v
• python -m pytest tests/test_knowledge_graph.py tests/test_knowledge_graph_extra.py tests/test_knowledge_graph_metadata.py -v
• python -m pytest tests/ -v

No production code changes.

Checklist

• Tests pass (python -m pytest tests/ -v)
• No hardcoded paths
• Linter passes (ruff check .)