Fix test issue to stabilise scoring through use of DFS search mode.
Randomised index-then-delete docs introduced by the test framework likely caused an imbalance in IDF scores across shards. Also made number of shards used in test a random number for added test coverage.
Closes#46174
* Search enhancement: pinned queries (#44345)
Search enhancement: - new query type allows selected documents to be promoted above any "organic” search results.
This is the first feature in a new module `search-business-rules` which will house licensed (non OSS) logic for rewriting queries according to business rules.
The PinnedQueryBuilder class offers a new `pinned` query in the DSL that takes an array of promoted IDs and an “organic” query and ensures the documents with the promoted IDs rank higher than the organic matches.
Closes#44074
markharwood