do not use $lookup for query with variables if only single variable set

[hallettj]

This allows a $vectorSearch to be the target of a remote join if the join forwards exactly one variable set.

[hallettj]

In addition to MongoDB Atlas not allowing $vectorSearch in sub-pipelines it looks like $vectorSearch does not substitute variable references in its arguments. So even after I added a workaround for the sub-pipeline issue yesterday the remote join use case still didn't work. In particular we had a problem with the queryVector argument.

There is a way to fix the variable issue with some more work: in the case of a query request with exactly one variable set rewrite the request to inline all variable values. That's another way to get the sub-pipeline workaround so it would replace the changes currently in this PR.