SOLR-18271: QueryElevation support for Combined Query feature

[ercsonusharma]

https://issues.apache.org/jira/browse/SOLR-18271

Description

Adds QueryElevationComponent support to the Combined Query feature, so that elevated docIds (via elevateIds / configured elevations) are honored when results from multiple subqueries are combined and merged across shards.

Solution

QueryElevationComponent.prepare now detects when it is operating on a CombinedQueryResponseBuilder and dispatches to a new prepareCombined path. That path runs the existing elevation preparation on each subquery's ResponseBuilder, then mirrors the resulting SortSpec and filters from a representative subquery onto the parent crb. This lets CombinedQueryComponent#mergeIds read the _elevate_ marker from each shard's sort_values_i during distributed merge, preserving elevation order across the combined result set.

Supporting changes:

• CombinedQueryComponent#prepare now propagates the debug flag (rbNew.setDebug(rb.isDebug())) to subquery response builders so debug info flows through.
• When debug is on, per-subquery queryBoosting debug entries are aggregated and re-attached to the parent crb under the same key.
• solrconfig-combined-query.xml (test config) adds a /search-elevate handler wired with the elevator (QueryElevationComponent) as a last-component, using elevate.xml.

Tests

Added two new tests as described below:

• CombinedQuerySolrCloudTest#testElevatedQueriesWithFacetAndHighlights - new SolrCloud integration test that issues a combined JSON query with two lexical subqueries plus elevateIds, faceting, and highlighting. Verifies elevated ids appear first in the expected order, and that facet counts and highlight snippets are still correct alongside elevation.

• DistributedCombinedQueryComponentTest#testElevatedQueriesWithFacetAndHighlights - new distributed test that passes lucene subqueries with elevateIds and debug=true. Verifies elevated order, facets, highlights, and that the merged debug response contains a queryBoosting entry with one element per subquery.

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
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• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change