Fix request building

[agourlay]

Fix several issues in the request building:

1. Prefetch builder result discarded - PrefetchQueryBuilder.query() returns a new builder, but the result was thrown away. Now the builder is properly chained: .query().params().limit().build().
2. Built prefetch never attached - The built PrefetchQuery is now added to the request via .prefetch(vec![prefetch]).
3. Wrong arg for hnsw_ef - Both occurrences of self.args.hnsw_ef_construct in the search path were changed to self.args.search_hnsw_ef (the parameter meant for search, not index construction).
4. search_params overwriting prefetch params - The prefetch branch now sets its own params on the prefetch query, and search_params is applied to the top-level request in both branches without conflict.

Also improving the Rust client to avoid this issue completely in the future qdrant/rust-client#276