* fix: apply score_threshold filtering after fusion queries in local mode
The local/memory client was not applying score_threshold filtering after
RRF and DBSF fusion operations. This caused query_points with prefetch
and fusion queries to return results below the specified score_threshold.
This fix adds score_threshold filtering after fusion results are computed,
matching the behavior of the remote Qdrant server.
* tests: simplify score threshold tests, add formula threshold test
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Co-authored-by: George Panchuk <george.panchuk@qdrant.tech>
