[crag] fix snapshots handling composed @graph
Summary:
Snapshot code was still coded against CompositeSolidDefinition checks without a fall-through so
- fix it
- re-arrange it to have a check.failed for unhandled node types
- rename getter to node since it has graphs and solids
Test Plan: added tests
Reviewers: cdecarolis, owen, yuhan, prha, sandyryza
Reviewed By: sandyryza
Differential Revision: https://dagster.phacility.com/D8943