[BUG][Python] Wrapper models for oneOf values include metadata fields in serialized responses

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When using the python-pydantic-v2 generator, defining a schema with additionalProperties whose values are oneOf produces an intermediate wrapper model for each dictionary value.

For the API definition below, the generator produces for followingApiResponse class:

class ApiResponse(BaseModel):
    """
    ApiResponse
    """ # noqa: E501
    message: StrictStr
    extra: Optional[Dict[str, ApiResponseExtraValue]] = Field(default=None, description="Map of extras")
    __properties: ClassVar[List[str]] = ["message", "extra"]

where the ApiResponseExtraValue class is generated as:

APIRESPONSEEXTRAVALUE_ONE_OF_SCHEMAS = ["ExtraAlpha", "ExtraBeta"]

class ApiResponseExtraValue(BaseModel):
    """
    ApiResponseExtraValue
    """
    # data type: ExtraAlpha
    oneof_schema_1_validator: Optional[ExtraAlpha] = None
    # data type: ExtraBeta
    oneof_schema_2_validator: Optional[ExtraBeta] = None
    actual_instance: Optional[Union[ExtraAlpha, ExtraBeta]] = None
    one_of_schemas: List[str] = Literal["ExtraAlpha", "ExtraBeta"]

    model_config = {
        "validate_assignment": True,
        "protected_namespaces": (),
    }

Returning a ApiResponse currently serializes all fields of the wrapper model, resulting in responses like:

{
    "oneof_schema_1_validator": null,
    "oneof_schema_2_validator": null,
    "actual_instance": {
        "type": "beta",
        "variant": "excel",
        "label": "Descargar",
        "downloadId": "68b84d830ed1dd568216e27a"
    },
    "one_of_schemas": ["ExtraButton", "ExtraTable"]
}

The desired output is only the actual payload:

{
    "type": "beta",
    "variant": "excel",
    "label": "Descargar",
    "downloadId": "68b84d830ed1dd568216e27a"
}

The generator creates a wrapper model for the extra property which was defined with oneOf value.
Wrapper fields like oneof_schema_1_validator, oneof_schema_2_validator, and one_of_schemas are metadata for the generator, not real API data. The generated code includes these fields in instance serialization, so FastAPI returns them in responses.

openapi-generator version

openapi-generator v7.15.0 (latest at the moment)
generator: python-fastapi (pydantic v2)
pydantic version: 2.11.7 (latest at the moment)
fastapi version: 0.116.1 (latest at the moment)

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Literal Bug Example
  version: 1.0.0
paths: {}
components:
  schemas:
    ApiResponse:
      type: object
      additionalProperties: false
      properties:
        message:
          type: string
          example: "Hello world"
        extra:
          description: "Map of extras"
          type: object
          additionalProperties:
            oneOf:
              - $ref: '#/components/schemas/ExtraAlpha'
              - $ref: '#/components/schemas/ExtraBeta'
      required:
        - message

    ExtraAlpha:
      type: object
      required: [type, foo]
      properties:
        type:
          type: string
          enum: ["alpha"]
        foo:
          type: string

    ExtraBeta:
      type: object
      required: [type, variant, label, url]
      properties:
        type:
          type: string
          enum: ["beta"]
        variant:
          type: string
          enum: ["excel"]
        label:
          type: string
          maxLength: 40
        downloadId:
          type: string
          maxLength: 50

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Manually converting metadata fields to ClassVar can remove them from the serialized response:

class ApiResponseExtraValue(BaseModel):
    """
    ApiResponseExtraValue
    """
    # data type: ExtraAlpha
    oneof_schema_1_validator: ClassVar[Optional[ExtraAlpha]] = None
    # data type: ExtraBeta
    oneof_schema_2_validator: ClassVar[Optional[ExtraBeta]] = None
    actual_instance: Optional[Union[ExtraAlpha, ExtraBeta]] = None
    one_of_schemas: ClassVar[List[str]] = Literal["ExtraAlpha", "ExtraBeta"]

    model_config = {
        "validate_assignment": True,
        "protected_namespaces": (),
    }

However, the actual_instance wrapper is still serialized:

{
    "actual_instance": {
        "type": "beta",
        "variant": "excel",
        "label": "Descargar",
        "downloadId": "68b943b1bb9793a88d289545"
    }
}

when the response should be

{
    "type": "download",
    "variant": "excel",
    "label": "Descargar",
    "downloadId": "68b84d830ed1dd568216e27a"
}

The suggested fix is:

• Only serialize the actual payload (actual_instance) when returning the model.
• Treat the wrapper fields (oneof_schema_1_validator, oneof_schema_2_validator, and one_of_schemas) as class-only metadata or remove them entirely.

Start with the supplied OpenAPI 3.0.3 declaration and generate the python-fastapi output with Pydantic v2. Inspect the generated ApiResponseExtraValue wrapper and reproduce serialization of the additionalProperties oneOf value. Done means the response contains only the actual payload, without wrapper metadata fields.