(refactor-input-source-to-thunk-4) Add checks to catch errors when new source types are added
For cases where we don't capture behavior differences in polymorphic methods, adding
invariants to ensure that we don't miss anything when source types are added.
Test Plan: BK
Reviewers: sandyryza, alangenfeld, yuhan
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5316