feat：add Graph store support

[bupt-lmy]

Summary

This PR brings the Java SDK Graph Store to Python-parity for OceanBase, including a dedicated graph_store.llm/* and graph_store.embedder/* override path, plus tests and docs updates.

What’s included

• Python-parity OceanBaseGraphStore

  • Implements entities + relationships schema (graph_entities / graph_relationships)
  • LLM tools extraction for entities/relations/deletions with robust parsing + fallbacks:
    • extract_entities
    • establish_relationships
    • delete_graph_memory
  • ANN-style entity resolution
    • Best-effort VECTOR(dims) column + CREATE VECTOR INDEX (HNSW/IVF)
    • Fallback to embedding_json brute-force when VECTOR is unavailable
  • Graph retrieval
    • Multi-hop traversal (id-based frontier expansion with cycle prevention behavior)
    • BM25 reranking with lightweight tokenizer (offline-friendly)

• GraphStore config parity

  • Added nested overrides:
    • graph_store.llm.* (provider/apiKey/model/baseUrl/temperature/maxTokens/topP)
    • graph_store.embedder.* (provider/apiKey/model/baseUrl/dims)
  • ConfigLoader supports env keys:
    • GRAPH_STORE_LLM_*, GRAPH_STORE_EMBEDDING_* (with embedder aliases)

• Core response parity

  • When GraphStore is enabled, relations is always returned for:
    • add, search, get_all (empty allowed; no null omission)

• Tests

  • Offline unit test for nested config parsing: GraphStoreConfigOverrideTest
  • Updated integration test usage patterns and assertions:
    • OceanBaseGraphStoreIT (env-gated)

• Docs

  • README structure updated and GraphStore section refreshed
  • Added GraphStore-specific .env examples for graph_store.llm/embedder overrides

Why

• Match Python powermem Graph Store behavior so benchmark/server output and graph workflows are consistent across languages.
• Enable running Graph Store with a dedicated model setup (e.g., Graph-specific Qwen LLM/embedding) without affecting Memory-level providers.
• Provide deterministic offline testing and safe, env-gated OceanBase integration tests.

Test plan

• Offline unit tests:
  • mvn test
• OceanBase integration tests (env-gated):
  • mvn -Dtest=OceanBaseGraphStoreIT test

Notes / Compatibility

• Vector index creation is best-effort and automatically falls back to brute-force when VECTOR/index features are not available in the target OceanBase cluster/version.
• BM25 tokenizer is a lightweight implementation (offline-friendly); ranking may differ slightly from Python’s jieba + rank_bm25 in some edge cases.

[Teingi]

这个文件存在是要解决什么问题？和python sdk 貌似不匹配？

[bupt-lmy]

GraphStore 的内存版实现，主要用于离线/单元测试，可以选择去除或保留