Fixes #35317: check hybrid search with a query, not the stored pipeline (backport 2.0 #35321)

[manerow]

Issue

Cherry-pick of PR #35321 (Fixes #35317)

Shared free-tier clusters run 2.0.x, so this backport is what makes the Semantic search health check stop reporting a false "Forbidden access" failure for them. It ships in 2.0.6.

What this PR changes

The same change as on main: the health check sends a minimal hybrid query with the RRF pipeline inline to the deployment's own chunk index, instead of reading the stored hybrid-rrf pipeline that a prefix-scoped search role is not allowed to read. See PR #35321 for the full explanation.

The cherry-pick had one conflict, in the imports of OpenSearchVectorServiceTest: 2.0 does not have main's context-memory tests, so their imports were dropped and the two Jackson imports the new tests need were added. Every other file applies main's change line for line.

How it was tested

On this branch: OpenSearchVectorServiceTest, SearchRepositoryBehaviorTest, SystemRepositoryEmbeddingsValidationTest and ReindexingOrchestratorTest (170 tests), plus VectorEmbeddingIntegrationIT against OpenSearch 3.4.0 (10 tests), all passing.

[gitar-bot]

Code Review ✅ Approved

🟡 Medium risk · The health check now executes a hybrid query instead of checking pipeline presence, so permission or query errors could incorrectly mark semantic search unavailable.

Health check for semantic search now sends a minimal hybrid query with the RRF pipeline inline to the deployment's own chunk index, instead of reading the stored hybrid-rrf pipeline that prefix-scoped roles cannot access. This backport of #35321 removes the false "Forbidden access" failure on shared free-tier 2.0.x clusters. No issues found.

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