There is a subtle bug found in this test case, where $facet validation (is every stage allowed in $facet) is only performed on its subpipelines before the $unionWith goes through view resolution, so we miss when a $unionWith-on-a-view has a stage in the view definition that's not allowed in $facet. There are few stages that are not allowed in $facet ($vectorSearch being one of them), so the blast radius of the bug here is somewhat small.

We should refactor $facet so that when LiteParsedFacet is being promoted to DocumentSourceFactor, the $facet subpipelines are also promoted from LiteParsedPipelines to Pipelines.

We also need to make sure its subpipelines finish resolving themselves: specifically for BF-41470 where the $unionWith-in-a-$facet needs to resolve its view pipeline before going through $facet subpipeline validation.