OpenSearch phase-2 gaps: neutral aggregations (histogram/date_histogram) + site-search highlights

Found during the phase-2 (dual-write, reads from OpenSearch) integration triage — see #36501. These are larger OS feature gaps, deferred from the phase-2 product-fix batch and related to the deferred OSSiteSearchAPI work (#34609 / follow-on to #35786).

1. OS neutral aggregations: histogram / date_histogram not mapped

Aggregation.fromSingleOS (dotCMS/src/main/java/com/dotcms/content/index/domain/Aggregation.java ~137-180) handles only terms/top_hits and returns null for everything else; fromOS drops nulls. Histogram and date_histogram are never produced. AggregationBucket (.../domain/AggregationBucket.java) has OS factories only for terms buckets — no histogram/date-histogram bucket overload. The ES path handles them (Aggregation.java ~101-106 → AggregationBucket.fromHistogram).

Fails (phase 2): SiteSearchWebAPITest.getAggregations_dateHistogram_keyAsEpochMillis, getAggregations_numericHistogram_keyAsNumber, getFacets_dateHistogram_exposesEpochMillisTime, getFacets_histogramAggregation_wrapsAsCountHistogramFacet.

Fix direction: add isHistogram()/isDateHistogram() branches to fromSingleOS + AggregationBucket.fromOS(HistogramBucket)/(DateHistogramBucket) normalizing the key to epoch-millis/numeric. Shared neutral DTO → also benefits OSSearchAPIImpl.

2. OS site-search highlights hardcoded empty

OSSiteSearchAPI.search() (dotCMS/src/enterprise/java/com/dotcms/enterprise/publishing/sitesearch/OSSiteSearchAPI.java ~283-286) sets ssr.setHighLight(new String[0]) with a TODO: the neutral SearchHit DTO does not carry per-field highlights yet. Fails (phase 2): ContentletIndexAPIImplTest.testSearch (the highlight-assertion half).

Related: #34609, #35786, #36501.

Start with Aggregation.java and AggregationBucket.java, then run the four named aggregation tests to trace the neutral OpenSearch mapping gaps. Read OSSiteSearchAPI.search() and ContentletIndexAPIImplTest.testSearch for the highlight path. Done means histogram/date_histogram results and site-search highlights are represented by the neutral APIs and the named tests pass.