Add native MMR support for dense search

[srimon12]

Summary

This PR adds native Qdrant MMR support for dense SEARCH.

Users can now write:

SEARCH docs SIMILAR TO 'retrieval systems' LIMIT 10
WITH { mmr_diversity: 0.5, mmr_candidates: 50 }

Changes

• added mmr_diversity and mmr_candidates to WITH
• wired dense SEARCH to use Qdrant NearestQuery(..., mmr=Mmr(...))
• wired dense SEARCH ... GROUP BY to use the same MMR path
• added explicit errors for unsupported combinations:
  • USING HYBRID
  • USING SPARSE
  • RECOMMEND
• updated docs and tests

Validation

Ran:

• uvx ruff check .
• pytest tests -q

Result:

• Ruff clean
• 519 passed