- Introduce a new `application/graphql-response+json` response format selectable via Accept header with distinct status codes for success and validation errors
- Require `Content-Type: application/json` for requests and drop legacy `application/graphql` format, causing 415 errors for unsupported types
- Accept header now supports `application/graphql-response+json` and `application/json`; unsupported Accept values return 406 and `*/*` defaults to the new format