Now that $_internalSearchIdLookup is built through the StageParams registry, its static DocumentSourceInternalSearchIdLookUp::createFromBson() is dead code: the stage is not registered via REGISTER_DOCUMENT_SOURCE or either StageParams bridge macro, and the only remaining reference to createFromBson() is a helper in document_source_visitor_docs_needed_bounds_test.cpp.

This ticket should remove the dead code and migrate the test helper to construct the stage via its public constructor instead.