The MongoExtensionPipelineDependencies object is passed to extension source stages (those with requiresInputDocSource=false) via apply_pipeline_suffix_dependencies. It currently exposes three queries: needs_metadata, needs_variable, and needs_whole_document. However, it does not expose which specific document fields the downstream pipeline references.

This prevents extension source stages (e.g., $_extensionSearch) from building field-limited queries to their external data source—for example, constructing a stored-source projection for mongot that returns only the fields actually consumed by the pipeline. 

DepsTracker::fields, a set of dotted field-path strings, are computed as part of the returned DepsTracker result from the dep analysis conducted on pipelines with an extension source stage.

A get_needed_fields function can be added to MongoExtensionPipelineDependenciesVTable that populates a MongoExtensionByteBuf* with a BSON array of dotted field-path strings representing DepsTracker::fields. The output is null when needs_whole_document is true (field enumeration is meaningless in that case). Implement the associated plumbing.

Add the corresponding unit and integration tests.