Generalize how queries on `_index` are handled at rewrite time (#52486)
Since this change refactors rewrites, I also took it as an opportunity to adrress #49254: instead of returning the same queries you would get on a keyword field when a field is unmapped, queries get rewritten to a MatchNoDocsQueryBuilder.
This change exposed a couple bugs, like the fact that the percolator doesn't rewrite queries at query time, or that the significant_terms aggregation doesn't rewrite its inner filter, which I fixed.
Closes#49254
*Most* of our parsing can be done without passing any extra context into
the parser that isn't already part of the xcontent stream. While I was
looking around at the places that *do* need a context I found a few
places that were declared to need a context but don't actually need it.
This commit replaces the SearchContext used in AbstractQueryTestCase with
a QueryShardContext in order to reduce the visibility of search contexts.
Relates #46523
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
