fix: validate CI command didn't show validation result

In !217 (merged) we converted the API validation result to boolean value to fix an eslint warning. Because the client code was written in JS, we didn't get any type errors and because the code was untested, we didn't get any test failure.

This MR fixes the small issue (removes the boolean conversion). It also converts the validation command to TS and adds integration tests for happy and sad path scenarios.

(We had to update our msw testing dependency because the new version allows us to look inside the POST request body in tests)

Related to #400 (closed)