[BUG][Python] oneOf generates properties with invalid Literal fields

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, if a schema property is defined as oneOf, the generator produces an intermediate model as follows:

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": (),
    }

Where one_of_schemas is defined as Literal[...]. This is not correct as Literal is a type and should be used as a type hint, not a value.
Pydantic (and FastAPI's serialize_response()) treats this like a real runtime field causing the error:

pydantic_core._pydantic_core.PydanticSerializationError: 
Unable to serialize unknown type: <class 'typing._LiteralGenericAlias'>

at runtime when serializing models.

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

    ExtraBeta:
      type: object
      properties:
        type:
          type: string
          enum: ["beta"]
        bar:
          type: integer
      required: [type, bar]

Generation Details

Steps to reproduce

Related issues/PRs

Suggest a fix

Start by running the supplied OpenAPI 3.0.3 YAML through the python-fastapi generator with Pydantic v2, then inspect the generated ApiResponseExtraValue model. Confirm how one_of_schemas is declared and reproduce serialization through FastAPI's serialize_response(). Done means the generated model no longer treats Literal[...] as a runtime field and serialization succeeds.