test: vector_search E2E with real embeddings + LangChain similarity_search

[polaz]

DB gate: G073 ✅ (VectorService → HNSW wiring complete)

Task

• E2E tests for vector_search() with real float embeddings via Docker
• Verify similarity_search() in LangChain adapter returns ranked results

[coderabbitai]

⚠️ Possible Duplicate Issue(s)

• fix: vector_search() always returns empty — E2E tests + LangChain similarity_search() #20

🔗 Related PRs

#27 - feat: similarity_search() for LangChain + upsert_relations() idempotency test [merged]

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