[MAINTENANCE] create Checkpoint endpoint contract test

[josectobar]

Add contract test for creating a Checkpoint in Cloud REST endpoint

• [ ] Description of PR changes above includes a link to an existing GitHub issue
• [x] PR title is prefixed with one of: [BUGFIX], [FEATURE], [DOCS], [MAINTENANCE], [CONTRIB]
• [x] Code is linted - run invoke lint (uses black + ruff)
• [x] Appropriate tests and docs have been updated

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[NathanFarmer]

These tests are no longer being added. Hopefully we can add more contract tests with the V1 API.