[BUG] [csharp] [netcore] Default Accept Headers do not respect 2xx vs non-2xx response code differences

Consider a schema such as:

"/process-definition/key/{key}/rendered-form": {
      "get": {
        "operationId": "getRenderedStartFormByKey",
        "tags": [
          "Process Definition"
        ],
        "summary": "Get Rendered Start Form",
        "description": "Retrieves  the rendered form for the latest version of the process definition which belongs to no tenant.\nThis method can be used to get the HTML rendering of a\n[Generated Task Form](https://docs.camunda.org/manual/latest/user-guide/task-forms/#generated-task-forms).",
        "parameters": [
          {
            "name": "key",
            "in": "path",
            "schema": {
              "type": "string"
            },
            "required": true,
            "description": "The key of the process definition (the latest version thereof) to be retrieved."
          }
        ],
        "responses": {
          "200": {
            "content": {
              "application/xhtml+xml": {
                "schema": {
                  "type": "string",
                  "format": "binary",
                  "description": "For `application/xhtml+xml` Responses, a byte stream is returned."
                },
                "examples": {
                  "example-1": {
                    "summary": "Status 200 Response",
                    "description": "A `/process-definition/key/anKey/rendered-form` HTML\n                                       GET response body providing the rendered (generated) form content.",
                    "value": "\u003cform class\u003d\"form-horizontal\"\u003e\n                                  \u003cdiv class\u003d\"control-group\"\u003e\n                                    \u003clabel class\u003d\"control-label\"\u003eCustomer ID\u003c/label\u003e\n                                    \u003cdiv class\u003d\"controls\"\u003e\n                                      \u003cinput form-field type\u003d\"string\" name\u003d\"customerId\"\u003e\u003c/input\u003e\n                                    \u003c/div\u003e\n                                  \u003c/div\u003e\n                                  \u003cdiv class\u003d\"control-group\"\u003e\n                                    \u003clabel class\u003d\"control-label\"\u003eAmount\u003c/label\u003e\n                                    \u003cdiv class\u003d\"controls\"\u003e\n                                      \u003cinput form-field type\u003d\"number\" name\u003d\"amount\"\u003e\u003c/input\u003e\n                                    \u003c/div\u003e\n                                  \u003c/div\u003e\n                                \u003c/form\u003e"
                  }
                }
              }
            },
            "description": "Request successful."
          },
          "400": {
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ExceptionDto"
                }
              }
            },
            "description": "Process definition has no form field metadata defined. See the\n[Introduction](https://docs.camunda.org/manual/latest/reference/rest/overview/#error-handling)\nfor the error response format."
          },
          "404": {
            "content": {
              "application/json": {
                "schema": {
                  "$ref": "#/components/schemas/ExceptionDto"
                }
              }
            },
            "description": "Process definition with given key does not exist.  See the\n[Introduction](https://docs.camunda.org/manual/latest/reference/rest/overview/#error-handling)\nfor the error response format."
          }
        }
      }
    }

The 200 response content is application/xhtml+xml and the 4xx response types are json.

The Code generator creates something like:

/// <summary>
        /// Get Rendered Start Form Retrieves the rendered form for a process definition. This method can be used to get the HTML rendering of a [Generated Task Form](https://docs.camunda.org/manual/latest/user-guide/task-forms/#generated-task-forms).
        /// </summary>
        /// <exception cref="Camunda.Http.Api.Client.ApiException">Thrown when fails to make API call</exception>
        /// <param name="id">The id of the process definition to get the rendered start form for.</param>
        /// <param name="cancellationToken">Cancellation Token to cancel the request.</param>
        /// <returns>Task of ApiResponse (System.IO.Stream)</returns>
        public async System.Threading.Tasks.Task<Camunda.Http.Api.Client.ApiResponse<System.IO.Stream>> GetRenderedStartFormWithHttpInfoAsync(string id, System.Threading.CancellationToken cancellationToken = default(System.Threading.CancellationToken))
        {
            // verify the required parameter 'id' is set
            if (id == null)
                throw new Camunda.Http.Api.Client.ApiException(400, "Missing required parameter 'id' when calling ProcessDefinitionApi->GetRenderedStartForm");


            Camunda.Http.Api.Client.RequestOptions localVarRequestOptions = new Camunda.Http.Api.Client.RequestOptions();

            String[] _contentTypes = new String[] {
            };

            // to determine the Accept header
            String[] _accepts = new String[] {
                "application/xhtml+xml",
                "application/json"
            };


            var localVarContentType = Camunda.Http.Api.Client.ClientUtils.SelectHeaderContentType(_contentTypes);
            if (localVarContentType != null) localVarRequestOptions.HeaderParameters.Add("Content-Type", localVarContentType);

            var localVarAccept = Camunda.Http.Api.Client.ClientUtils.SelectHeaderAccept(_accepts);
            if (localVarAccept != null) localVarRequestOptions.HeaderParameters.Add("Accept", localVarAccept);

            localVarRequestOptions.PathParameters.Add("id", Camunda.Http.Api.Client.ClientUtils.ParameterToString(id)); // path parameter


            // make the HTTP request

            var localVarResponse = await this.AsynchronousClient.GetAsync<System.IO.Stream>("/process-definition/{id}/rendered-form", localVarRequestOptions, this.Configuration, cancellationToken).ConfigureAwait(false);

            if (this.ExceptionFactory != null)
            {
                Exception _exception = this.ExceptionFactory("GetRenderedStartForm", localVarResponse);
                if (_exception != null) throw _exception;
            }

            return localVarResponse;
        }

and the SelectHeaderAccept (ClientUtils.cs) method does:

        public static String SelectHeaderAccept(String[] accepts)
        {
            if (accepts.Length == 0)
                return null;

            if (accepts.Contains("application/json", StringComparer.OrdinalIgnoreCase))
                return "application/json";

            return String.Join(",", accepts);
        }

As a result the endpoint fails because the Request Accept header is application/json.

Should the generator understand the 2xx codes vs error codes and excepted response typed?

Start with the generated C# client behavior described in the issue and inspect ClientUtils.cs, especially SelectHeaderAccept, along with the generator path that assembles response media types. Confirm how 2xx and error responses are represented, then verify that generated clients negotiate application/xhtml+xml for success and application/json for errors.