Summary:
PgVector provides the capability to configure the expansion factor (ef) parameter for search operations.
This is accomplished by setting the `ybhnsw.ef_search` variable, which is subsequently incorporated into the vector read options.

To fully support this functionality, we need to implement additional logic that propagates the specified `ef_search` value to the underlying USearch library.
This ensures that the configured expansion factor is properly utilized during the search process.

Original commit: c7c5a4298be4923147b72c11e071196604f8b951 / D43355

Jira: DB-16297

Test Plan: PgVectorIndexTest.EfSearch/*

Reviewers: arybochkin, rthallam

Reviewed By: arybochkin, rthallam

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D43560