Implement backend vector search for graph drivers

[cpdata]

Summary

• add a GraphDriver.vector_search fallback and delegate graph_vector_search to driver-specific implementations
• execute cosine scoring inside Memgraph and Neo4j via Cypher, updating test doubles to assert the new queries
• record the closure of meshmind-82 across the tracker collateral and changelog

Testing

• pytest meshmind/tests/test_graph_retrieval.py meshmind/tests/test_memgraph_driver.py

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https://chatgpt.com/codex/tasks/task_b_68f5e9e124fc832183703534eb19dd42