The parsers in document_source_search.cpp and document_source_search_meta.cpp will be updated as follows:

REGISTER_LITE_PARSED_DOCUMENT_SOURCE_FALLBACK(
    {search|searchMeta},
    [](const NamespaceString& nss,
       const BSONElement& spec,
       const LiteParserOptions& options) -> std::unique_ptr<LiteParsedDocumentSource> {
   // Check if the extension is loaded on this shard, tassert otherwise.
        return {Search|SearchMeta}LiteParsed::parse(nss, spec, options);
    },
    AllowedWithApiStrict::kNeverInVersion1,
    &feature_flags::gFeatureFlagSearchExtension);

Unit Tests

• Fallback Parser Registration (document_source_search{_meta}_test.cpp)
• Parser tasserts when featureFlagSearchExtension is true but the extension is not loaded.
• Parser uses the fallback legacy implementation when featureFlagSearchExtension is false.

Integration Tests

• IFR Flag Toggle
• Extension loaded + featureFlagSearchExtension=true uses the extension.
• Extension loaded + featureFlagSearchExtension=false uses the fallback.
• No extension loaded uses the fallback regardless of featureFlagSearchExtension.
• Flipping the flag at runtime correctly toggles between legacy and extension implementations.