When extensions are sent from the router to the shard, they have already been expanded on the router. However, after being serialized, the shard cannot recognize that the stage has already been expanded.

To prevent an unnecessary additional expansion call on the shard, we should add a DocumentSourceExtensionOptimizable constructor that "skips" DocumentSourceExtensionExpandable entirely, given that the parseNode passed through only expands to a single, extension-allocated astNode.