[REQ] [Python-Fastapi] Add status_code in route decorators based on OpenAPI 2xx

Is your feature request related to a problem? Please describe.

When using the python-fastapi generator, the generated endpoints do not set the status_code argument on the FastAPI route decorators, even when the OpenAPI specification clearly defines a specific success status code.

For example, given an OpenAPI operation like:

paths:
  /example:
    post:
      summary: Create example
      operationId: createExample
      responses:
        '201':
          description: Created
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ExampleResponse'
        '400':
          description: Bad Request
        '500':
          description: Server Error

the generated FastAPI router looks roughly like this:

@router.post(
    "/example",
    responses={
        201: {"model": ExampleResponse, "description": "Created"},
        400: {"model": Message, "description": "Bad Request"},
        500: {"model": Message, "description": "Server Error"},
    },
    summary="Create example",
    response_model_by_alias=True,
)
async def create_example(
    body: ExampleRequest = Body(None),
) -> ExampleResponse:
    return await BaseExampleApi.subclasses[0]().create_example(body)

FastAPI’s default HTTP status code for this route is 200, because the decorator does not specify status_code. This is inconsistent with the OpenAPI spec, which documents 201 as the success status. According to the FastAPI documentation (https://fastapi.tiangolo.com/reference/status/), the status_code argument should be used to make the runtime behavior match the documented success status.

Describe the solution you'd like

I would like the python-fastapi generator to set the status_code argument on the generated route decorator when the OpenAPI spec clearly defines a primary success code.

For example, for the previous spec the generator could emit:

@router.post(
    "/example",
    responses={
        201: {"model": ExampleResponse, "description": "Created"},
        400: {"model": Message, "description": "Bad Request"},
        500: {"model": Message, "description": "Server Error"},
    },
    summary="Create example",
    response_model_by_alias=True,
    status_code=status.HTTP_201_CREATED,
)
async def create_example(
    body: ExampleRequest = Body(None),
) -> ExampleResponse:
    ...

This way:

The runtime HTTP status code matches the OpenAPI documentation.
The generated code follows FastAPI’s recommended pattern of using constants from fastapi.status as described in https://fastapi.tiangolo.com/reference/status/.
One simple heuristic could be:

If there is exactly one 2xx response defined, use that as the status_code.
Optionally, allow an explicit override via a vendor extension such as x-default-status-code on the operation.

Describe alternatives you've considered

• Manual edits after generation: After each code generation, manually edit the generated FastAPI router modules to add status_code=status.HTTP_201_CREATED (or similar) to each decorator. This is error-prone and must be repeated every time the code is regenerated.

Start with the python-fastapi generator's route decorator generation and its handling of OpenAPI 2xx responses. Determine how a primary success code should be selected, then add status_code using fastapi.status so generated runtime behavior matches the specification. Confirm the generated example uses HTTP_201_CREATED for the single 201 response.