feat(query): make hybrid_search ranker optional with RRF default

[marlon-costa-dc]

Summary

• Makes the ranker parameter in hybrid_search an Option<Box<dyn BaseRanker>>.
• If None is provided, it falls back to options.ranker or defaults to RrfRanker with k=60.0.
• This simplifies the API for the common case where users just want standard RRF ranking.

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