Update unit tests to use a more recent Elasticsearch client version

[miguelgrinberg]

The unit tests are currently designed for Elasticsearch clients up to 8.15.x. The 8.16 client introduced some changes in how hybrid search requests are made, using a retriever query instead of the knn query in older clients. The unit tests have assertions that depend on the requests issued by the 8.15 and older clients. These need to be updated, or better yet, the tests should not depend on what requests the low-level issues.