Extensions aren’t allowed in hybrid search stages ($rankFusion and $scoreFusion) until featureFlagExtensionsInsideHybridSearch is enabled. In parseAndValidate{Rank|Scor}edSelectionPipelines:

std::map<std::string, std::unique_ptr<Pipeline>> parseAndValidate{Rank|Scor}

edSelectionPipelines(...) {
// ...
if (!feature_flags::featureFlagExtensionsInsideHybridSearch.isEnabled() && liteParsedPipeline.hasExtensionSearchStage()

// ...
}

Integration Tests:

• $search/$searchMeta inside hybrid search with the appropriate flag disabled.
  • IFR kickback fires and the query retries.
  • The appropriate metric is incremented.
• $search/$searchMeta inside hybrid search with the appropriate flag enabled.
  • No IFR kickback or metric increment.