Proposal: optional Milvus backend for VectorDatabaseCore

[zc277584121]

Hi maintainers,

I noticed the Nexent SDK has a VectorDatabaseCore abstraction and documentation/comments indicating multiple backend possibilities, including Milvus. The current public wiring appears to expose Elasticsearch / DataMate paths, while Milvus is not yet available as a selectable backend.

Would you be open to adding Milvus as an optional vector database backend under the existing VectorDatabaseCore direction?

A possible scoped approach:

1. Keep the current Elasticsearch / DataMate behavior unchanged.
2. Clarify the backend selection surface for vector database implementations.
3. Add a Milvus implementation behind an optional dependency and config value.
4. Support Milvus Lite for local development and Milvus server / Zilliz Cloud for production deployments.
5. Add focused SDK tests for create/upsert/search/delete behavior without requiring the full application stack.

This seems aligned with the existing vector database abstraction, but I wanted to confirm the preferred route before proposing code. Would this fit Nexent's roadmap?