test: decouple ContentletIndexAPIImplTest from ElasticSearch internals (phase-aware, vendor-neutral) - Make it possible to run MainSuite Tests in all 3 phases

Description

ContentletIndexAPIImpl is the vendor-neutral router of the ES→OpenSearch migration, but its integration test ContentletIndexAPIImplTest (dotcms-integration/src/test/java/com/dotcms/content/elasticsearch/business/ContentletIndexAPIImplTest.java, 1261 lines, 14 @Test) is still coupled to ElasticSearch internals.

Running the suite under a non-zero migration phase (verified at phase 3 via the new opensearch-phase-suite profile) makes 6 of 14 tests fail — most of them purely because of ES-specific assertions/helpers, not product defects.

./mvnw verify -pl :dotcms-integration -Dcoreit.test.skip=false \
  -Dopensearch.phase=3 \
  -Dit.test=ContentletIndexAPIImplTest

In scope (test decoupling — fix here):

1. Replace the ES-direct search helper indexSearch() (~L1088-1110: RestHighLevelClientProvider + SearchRequest/QueryBuilders/SearchSourceBuilder/SearchResponse/SearchHits) with the vendor-neutral read path (SearchAPI / ContentletAPI.search / ContentSearchResponse) so the read-back hits whichever store the active phase routes to.
2. Replace @Test(expected = ElasticsearchStatusException.class) (~L1257, testGetIndexDocumentCountWithInvalidIndexNameFails) with the neutral exception the router throws (DotRuntimeException wrapping the OS index-not-found). Observed under phase 3: expected ElasticsearchStatusException but was DotRuntimeException.
3. Make the .os naming assertion in activateDeactivateIndex phase-aware (observed ComparisonFailure: expected:<live_…[.os]> but was:<live_…[]>).
4. Re-evaluate testGetIndexDocumentCountSuccess and createContentIndexAndDelete (AssertionError) against the store the active phase uses.
5. Remove the 8 org.elasticsearch.* imports (L80-87) once neutralized.

Out of scope (genuine product gaps — file as separate bug issues):

• optimize → index_not_found_exception looking up the index without the .os suffix (optimize-path routing/naming gap under phase 3).
• testSearch → MissingRequiredPropertyException: 'Builder.<variant kind>' parsing sitesearch index settings (OpenSearch client deserialization).
• Test_TrackHits_SearchCount (in ESContentFactoryImplTest) → expected:<60> but was:<30>: the OS search path doesn't honor ES_TRACK_TOTAL_HITS like ES.

Acceptance Criteria

• ContentletIndexAPIImplTest passes under phase 0 (normal profile, ES) and phase 3 (-Dopensearch.phase=3).
• No org.elasticsearch.* imports remain in the file.
• Read-back and exception/naming assertions are vendor-neutral / phase-aware.
• The 3 out-of-scope product gaps are filed as separate issues and linked here.

Additional Context

Blast-radius experiment (local, phase 3 / OS-only, opensearch-phase-suite): ContentletIndexAPIImplTest 8/14; rest of the high-level flow ~93% pass (ContentletCheckInTest 11/11, ESContentletAPIImplTest 87/89, ESContentFactoryImplTest 39/42) — confirming ES coupling is concentrated in this one class, not spread across the suite.

Related: PR #36268 / issue #36266 (added the opensearch-phase-suite profile and the phase-aware test bootstrap).