Based on the spec changes for error responses in Gerrit patch 770444, new OpenAPI validation tests are needed to cover the error responses:
- for the 400 case (Bad Request, validation errors) ✅️ Done
- for the 404 case (Not Found) ✅️ Done
- for the 500 case (Unexpected error) ❌️ no spec validation test, as the response can't be triggered from outside