Problem Statement:
Any Source stages in $unionWith sub-pipelines that reference unavailable metadata (e.g., {$meta: "textScore"}) do not throw validation errors during dependency analysis. Instead, they silently return empty documents at runtime.

Analysis:
DocumentSourceUnionWith::getDependencies() creates a DepsTracker with the default constructor, which uses NoMetadataValidation() and disables metadata validation. This means when stages call setNeedsMetadata(), validation is skipped.

Additionally, source stages short-circuit early in {{ pipeline_d.cpp::buildInnerQueryExecutor() }}, so they never reach the validateMetaDependencies() call in buildInnerQueryExecutorGeneric(). This means the only validation opportunity is during optimization phase.

Therefore, source stages (including) in $unionWith sub-pipelines referencing missing metadata do not fail validation.