Vectorstore: use a retriever query for hybrid search

[miguelgrinberg]

Fixes #2651

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[pquentin]

While this looks good, can you please explain why we can no longer disable RRF with AsyncDenseVectorStrategy(..., rrf=False)? Should we change the annotation of that rrf parameter from Union[bool, Dict[str, Any]] to Dict[str, Any]?

[miguelgrinberg]

I did not consider the case of rrf = False, I just assumed that rrf = False means that hybrid search is off, but now I realize that we have two booleans.

I'm not sure what would be the expected response when having hybrid search enabled and RRF disabled. Does that even make sense? I guess I need to try it out to see what response I get.

[miguelgrinberg]

With rrf=False we have to use a standard query, since the retriever query can only combine results from multiple searches with RRF.

[miguelgrinberg]

Okay, new revision of this fix. When the user asks for hybrid=True and rrf=False we use a regular query with knn and query sections in the body. Only when rrf=True or rrf={<rrf-options>} we use the new retriever query.