fix(rag): normalize Milvus L2 retrieval scores

[liugy789]

Summary

• Pass the configured metric type to the Milvus SearchReq.
• Normalize L2 distances into higher-is-better scores.
• Apply scoreThreshold after score normalization.
• Add regression tests for score conversion, metric propagation, and thresholding.

Why

Milvus L2 search returns distances where smaller values indicate greater similarity. AgentScope retrieval uses a higher-is-better score contract, so raw L2 distances must be adapted before they are exposed as document scores.

This PR uses 1 / (1 + distance) as an AgentScope-side normalization formula. This formula is not a Milvus-defined conversion.

Validation

• MilvusStoreTest: 65 tests passed.
• RAG simple module: 449 tests passed.
• Spotless check passed.
• git diff --check passed.

References

• Milvus Metric Types
• Milvus Basic Vector Search
• Milvus Java IndexParam API

Fixes #3069

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[dailingtao]

LGTM