On SERVER-105337 and SERVER-106365 we added support for setting the mixed-schema flag on the global catalog, which is necessary for proper routing of queries on sharded time-series collections containing mixed-schema buckets.

However no existing test was failing for this reason. We should add a test that asserts query plans are created according to the mixed-schema flag in the global catalog.

This could for example be a generic passthrough test that asserts the explain output has the expected $type clauses when the mixed schema flag is set, which should cover the scenario where the router is the one that has to add the clauses.