[BUG][Java] Response content type always set to "application/problem+json" when multiple json types exist

Description

When the OpenAPI spec contains two json types (application/json and application/problem+json), the generated code always chooses "application/problem+json" as the content type.

openapi-generator version

org.openapi.generator version "5.1.1"

OpenAPI declaration file content or url

"503": {
  "description": "Service Unavailable",
  "content": {
    "application/problem+json": {
      "schema": {
        "$ref": "#/components/schemas/RFC7807Response"
      }
    }
  }
},
"default": {
  "description": "default response",
  "content": {
    "application/json": {
      "schema": {
        "$ref": "#/components/schemas/DeviceSessionModel"
      }
    }
  }
}

Command line used for generation

java -jar openapi-generator-cli-5.0.0.jar generate -g java -i swagger.json

Steps to reproduce

Regardless of the ordering in the OpenAPI declaration file, application/problem+json appears first in the array & is set as the content type. Look at these lines in the generated api:

        final String[] localVarAccepts = { 
            "application/problem+json", "application/json"
        };
        final List<MediaType> localVarAccept = apiClient.selectHeaderAccept(localVarAccepts);

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/6779

Suggest a fix/enhancement

Not specifying "application/json" as a content type causes us trouble when trying to identify a RESTful request.

Reproduce the issue with the supplied OpenAPI declaration and Java generation command, then trace how the Java generator produces the generated API's localVarAccepts array. Verify the content-type ordering and selection when application/json and application/problem+json are both present, including reversed declaration order; done means the generated client no longer always prioritizes application/problem+json and regression coverage confirms the behavior.