fix: Apply discriminator literals to allOf subtypes without field reference

[staging-devin-ai-integration]

Summary

Fixes koxudaxi#2528 - OpenAPI polymorphism with discriminator and allOf not working

When a schema has a discriminator and subtypes inherit from it via allOf, the discriminator literal types (e.g., Literal['cat']) should be applied to the subtypes even when the parent schema isn't referenced by any field.

This follows the OpenAPI 3.1 spec example where Pet has a discriminator and Cat/Dog/Lizard extend it via allOf.

Problem

Previously, discriminator literal types were only applied when:

1. A field referenced the parent schema (e.g., PetContainer.pet: Pet)
2. This triggered the __apply_discriminator_type processing

But when schemas only use allOf inheritance without a containing field (the example from the spec), the discriminator literals were not applied.

Input Schema:

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

Before (incorrect):

class Cat(Pet):
    name: str | None = None

After (correct):

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

Solution

Added a new method _apply_allof_discriminator_to_models in OpenAPIParser that:

1. Tracks discriminator subtypes during schema collection in _collect_discriminator_schemas
2. Stores pending discriminator info mapping subtype refs to (discriminator_value, property_name)
3. Applies discriminator literal types to subtype models during module processing via override of _process_single_module
4. Handles field name transformations (e.g., snake_case) and aliases correctly
5. Supports both single alias and validation_aliases (multiple aliases) patterns

The fix leverages the existing _discriminator_schemas and _discriminator_subtypes dictionaries which were already being populated but not fully utilized.

Testing

Added test case test_main_openapi_discriminator_allof_standalone with input schema and expected output matching the OpenAPI 3.1 spec example.

All 29 existing discriminator-related tests continue to pass.

Notes

• Link to Devin session: https://staging.itsdev.in/sessions/8e9206754c894fb8b9ac38b9cdbe69c3

[staging-devin-ai-integration]

🤖 Devin AI Engineer

I'll be helping with this pull request! Here's what you should know:

✅ I will automatically:

• Address comments on this PR. Add '(aside)' to your comment to have me ignore it.
• Look at CI failures and help fix them

Note: I can only respond to comments from users who have write access to this repository.

⚙️ Control Options:

• Disable automatic comment and CI monitoring