fix: clean up graph search tools and results

[wayyoungboy]

Summary

Closes #1004.

This PR fixes the graph search/tool issues reported in #1004.

Changes:

• aligns the structured relations tool name with the regular establish_relationships tool name
• passes noop alongside entity extraction and relation extraction tools so the LLM can opt out
• deduplicates graph search triples across overlapping seed-node traversals
• includes the BM25 rerank score in returned graph search results
• adds unit coverage for the tool name, noop tools, duplicate removal, and score exposure

Validation

• python -m pytest tests/unit/test_oceanbase_graph.py -q
• python -m pytest tests/unit -q
• python -m flake8 src/powermem/storage/oceanbase/oceanbase_graph.py src/powermem/prompts/graph/graph_tools_prompts.py tests/unit/test_oceanbase_graph.py --select=F601,F821,E999
• git diff --check