feat: add keyword and hybrid retrieval for milvus

[jonasHanhan]

Summary

• Add keyword-only and hybrid retrieval modes for Milvus.
• Expose search_mode and keyword_top_k via query params.
• Keep existing vector retrieval and reranker flow intact.

Motivation

Issue #490: align query-test behavior with agent keyword retrieval by supporting keyword and hybrid search.

Testing

• Manual: UI verification with a test document; keyword vs vector vs hybrid produce expected differences.
• Automated: not run (not requested).