OpenAPI mock response generator

Synthesize a JSON response body for one operation + status code in an OpenAPI 3.x or Swagger 2.x spec. Locate the operation by operationId or method+path. Pick the response by explicit `status`, else the first 2xx, else the first documented response. Generation precedence: operation-level `example`/`examples` > schema `example` > recursive type-inferred walk (objects walk properties, arrays return [mock(items)], enums return the first value, $ref is surfaced literally).