Incorrect code mapping when "Default" directive is used.

Hello! I'm trying out the new 6.0 release and noticed it generated some invalid Python-FastAPI code. Here is the offending line of code:

https://github.com/OpenAPITools/openapi-generator/blob/5307a8dafd5cb45e7425eb6ebdc012dedcc82be4/modules/openapi-generator/src/main/resources/python-fastapi/api.mustache#L34

And here is the OpenAPI.yaml that was used as input:

https://github.com/OAI/OpenAPI-Specification/blob/aa91a19c43f8a12c02efa42d64794e396473f3b1/examples/v3.0/petstore-expanded.yaml#L51

And lastly here is the invalid python code that was generated:

    responses={
        200: {"model": Pet, "description": "pet response"},
        200: {"model": Error, "description": "unexpected error"},
    },

In the openapi.yaml file there was a response model for status code 200 and a response model for "Default" (everything else). The code generator interpreted this "Default" as a 200 which resulted in a python dictionary with 2x keys for 200, which is invalid.

Start with modules/openapi-generator/src/main/resources/python-fastapi/api.mustache at the referenced response mapping and compare it with the Default response in petstore-expanded.yaml at line 51. Regenerate the Python-FastAPI output from that specification and confirm the result preserves the explicit 200 response while handling Default without producing duplicate dictionary keys.