Schema validation displays "expected $ref" instead of more helpful "Unexpected field `foo` expected a $ref or Schema."
### Checklist
- [ ] Validation: I believe my source definition is inval…
During call to `$c->openapi->validate` existense of `in: body` parameter is currently verified via `$c->req->json`, and `json` method returns something only if `$c->req` is a valid JSON.
Relevant c…