feat: schema helpers + traverse (get_labels, get_edge_types, traverse)

[polaz]

Summary

• get_labels() → list[LabelInfo] — list all node labels in schema
• get_edge_types() → list[EdgeTypeInfo] — list all edge types in schema
• traverse(start_node_id, edge_type, direction, max_depth) → TraverseResult — graph traversal RPC
• LabelInfo, EdgeTypeInfo, PropertyDefinitionInfo, TraverseResult result types
• 15 mock-based unit tests, 5 integration tests (outbound/inbound traverse, label/edge-type listing)

[coderabbitai]

🔗 Related PRs

#15 - fix: CoordiNode Cypher compatibility — add_graph_documents, type/label, MATCH+CREATE [merged]

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