Extend dependency analysis so extensions can see SEARCH_META usage by (1) implementing a helper like PipelineRewriteContext::hasReferenceToVariableInPipelineSuffix(Variables::Id) using addVariableRefs / Variables::hasVariableReferenceTo over the pipeline suffix, and (2) exposing this through MongoExtensionPipelineDependencies and apply_pipeline_suffix_dependencies so the $search extension can reliably detect downstream SEARCH_META references (including in sharded/merge pipelines) and decide when to produce SEARCH_META. Add appropriate unit and integration tests.