Model Composition with ‘allOf’ Is Missing Fields

Describe the bug
Using a composition model (https://spec.openapis.org/oas/v3.0.4.html#models-with-composition) does not generate the target type but only the underlying type. For a type A which consists of the fields of B and some additional fields, only B is generated. Furthermore, A is replaced by B.

The spec below generates the following function:

def sync_detailed(
    *,
    client: AuthenticatedClient | Client,
    body: Base,
) -> Response[Any]: ...

I would expect that the model Network is generated and used instead of Base. The Network model should include the field of Base and the field non_base_field.

OpenAPI Spec File

openapi: 3.0.0
info:
  title: Service
  version: 0.0.0
paths:
  /widgets:
    get:
      responses:
        "200":
          description: The request has succeeded.
          content:
            application/json:
              schema:
                nullable: true
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: "#/components/schemas/Network"
components:
  schemas:
    Base:
      type: object
      required:
        - base_field
      properties:
        base_field:
          type: string
    Network:
      type: object
      required:
        - non_base_field
      properties:
        non_base_field:
          type: string
      allOf:
        - $ref: "#/components/schemas/Base"

Desktop (please complete the following information):

• OS: Linux
• Python Version: 3.13.11
• openapi-python-client version: 0.28.1

Additional context
Add any other context about the problem here.

Start by running the supplied OpenAPI spec and compare the generated Network and Base models and the sync_detailed signature. Trace how allOf schemas are handled during model and endpoint generation. Done means Network is generated with both fields, the endpoint uses Network instead of Base, and a regression test covers this composition.