Function fill_missing_response_descriptions 

pub fn fill_missing_response_descriptions(openapi: &mut OpenApi)

Fills empty response descriptions so utoipa emits OpenAPI’s required field.

Uses the HTTP reason phrase for known status codes. Default, range, unknown status codes, and reusable responses without a status use “Response”. Covers paths, webhooks, callbacks, and reusable components. References and non-empty product descriptions stay intact. Applying this twice has no effect.