Fix OpenAPI discriminator with allOf when no container references parent

[staging-devin-ai-integration]

Summary

This PR fixes issue koxudaxi#2528 - OpenAPI polymorphism with discriminator and allOf not working.

Problem

When a discriminator is used with allOf (as described in the OpenAPI spec), the discriminator literal types were not applied to subtype schemas if there was no container schema referencing the parent.

For example, given:

Pet:
  discriminator:
    propertyName: petType
    mapping:
      cat: '#/components/schemas/Cat'
Cat:
  allOf:
    - $ref: '#/components/schemas/Pet'
    - type: object
      properties:
        name:
          type: string

The generated Cat class would not have pet_type: Literal['cat'] unless there was another schema like PetContainer with a field referencing Pet.

Solution

• Added _get_discriminator_info_for_subtype() hook method in the base Parser class that returns None by default
• Overrode this hook in OpenAPIParser to return discriminator info for known subtypes (schemas that use allOf to inherit from a discriminated parent)
• Extended __apply_discriminator_type() in the base parser to also process models that are known subtypes, applying the discriminator literal types even when not referenced by a field

Changes

• src/datamodel_code_generator/parser/base.py: Added hook method and extended discriminator processing logic
• src/datamodel_code_generator/parser/openapi.py: Implemented hook to provide discriminator info for subtypes
• tests/main/openapi/test_main_openapi.py: Added test case
• tests/data/openapi/discriminator_allof_no_container.yaml: Test input schema (exact example from OpenAPI spec)
• tests/data/expected/main/openapi/discriminator/allof_no_container.py: Expected output

Testing

All discriminator-related tests pass:

pytest tests/main/openapi/test_main_openapi.py -k discriminator
# 29 passed

Example

Input schema (from OpenAPI spec example):

Pet:
  type: object
  required:
    - petType
  properties:
    petType:
      type: string
  discriminator:
    propertyName: petType
    mapping:
      cat: '#/components/schemas/Cat'
      dog: '#/components/schemas/Dog'
      lizard: '#/components/schemas/Lizard'
Cat:
  allOf:
    - $ref: '#/components/schemas/Pet'
    - type: object
      properties:
        name:
          type: string

Generated output (now correct):

class Pet(BaseModel):
    pet_type: Annotated[str, Field(alias='petType')]

class Cat(Pet):
    name: str | None = None
    pet_type: Literal['cat'] = Field(..., alias='petType')

[staging-devin-ai-integration]

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