If a $sort (on metadata fields) stage is present directly after an $_internalSearchIdLookup stage, the $sort stage can be safely pushed down before the $_internalSearchIdLookup. Conceptually, it should not matter if the results are sorted before or after they are converted from id's to full documents, as long as the $sort criteria is on metadata (because the metadata fields should all be present before lookup).

We want to push down the $sort when possible, because other stages before $_internalSearchIdLookup maybe be able to optimize it away. For example, a $search or $vectorSearch, whose results are sorted on the metadata 'searchScore' or 'vectorSearchScore', are able to remove a $sort that is sorted on the same metadata criteria.

Make sure to add test cases to pipeline_test.cpp, that manually add a $_internalSearchIdLookup stage before a $sort on metadata, then check that after pipeline optimization, the $sort appears before the $_internalSearchIdLookup.