[core] Resolve oneOf discriminator property type from child schemas

[Ignacio-Vidal]

What

getDiscriminatorPropertyType resolved the discriminator property's type only from the oneOf/anyOf schema's own properties, defaulting to "string" otherwise. For a pure oneOf/anyOf interface whose discriminator property lives on a shared base the children inherit via allOf, this produced a String getter that clashed with the enum the subtypes actually expose.

This adds a fallback (in DiscriminatorUtils) that resolves the type from the mapped child schemas, chasing the property through each child's allOf members. It only fires when the previous code would have returned "string", so existing specs are unaffected — regenerating the full sample suite produces no diffs.

Scenarios covered

Discriminator property declared…	Resolves to	Test
As a $ref on the schema's own properties (unchanged path)	the referenced type (e.g. PetType)	DiscriminatorUtilsTest, DefaultCodegenTest
Only on a shared base the oneOf children inherit via allOf	PetType (from children)	DiscriminatorUtilsTest, DefaultCodegenTest
Only on a shared base the anyOf children inherit via allOf	PetType (from children)	DiscriminatorUtilsTest
On a grandparent reached through multi-level allOf	PetType (recursive descent)	DiscriminatorUtilsTest
As a plain inline string (no $ref, no enum)	"string" (fallback, unchanged)	DiscriminatorUtilsTest, DefaultCodegenTest

Simples example where the discriminator is in the base object that is separate from the schema that contains oneOf

PetRequest:
      type: object
      oneOf:
        - $ref: '#/components/schemas/CatRequest'
        - $ref: '#/components/schemas/DogRequest'
      discriminator:
        propertyName: petType
        mapping:
          CAT: '#/components/schemas/CatRequest'
          DOG: '#/components/schemas/DogRequest'

    # Shared base WITHOUT a discriminator, so children flatten it via allOf
    # instead of extending it (which would create a cyclical reference).
    PetBase:
      type: object
      required:
        - petType
        - name
      properties:
        petType:
          $ref: '#/components/schemas/PetType'  # ---> This would be duplicated into PetRequest without this PR refactor
        name:
          type: string

    CatRequest:
      type: object
      allOf:
        - $ref: '#/components/schemas/PetBase'
        - type: object
          required:
            - indoor
          properties:
            indoor:
              type: boolean

    DogRequest:
      type: object
      allOf:
        - $ref: '#/components/schemas/PetBase'
        - type: object
          required:
            - trained
          properties:
            trained:
              type: boolean

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---

Summary by cubic

Fixes discriminator type resolution for oneOf/anyOf interfaces by deriving the property type from child schemas (via allOf) with cycle-safe recursion. Getter types now use the correct enum (e.g., PetType) instead of "string", avoiding interface vs subtype mismatches.

• Bug Fixes
  • Reworked DiscriminatorUtils.getDiscriminatorPropertyType(OpenAPI, ...) to check the schema’s own $ref first, then fall back to oneOf/anyOf children via getDiscriminatorPropertyTypeFromChildren and getDiscriminatorSchemaDeep (supports multi-level allOf).
  • Added visited-schema guard to getDiscriminatorSchemaDeep to handle cyclic allOf graphs safely.
  • Updated DefaultCodegen.getDiscriminatorPropertyType to delegate to the util, map to model names, and keep the "string" fallback; added DiscriminatorUtilsTest, expanded DefaultCodegenTest, and new YAMLs (oneOf/anyOf, nested base, cyclic, string fallback). Samples regenerate with no diffs.

^{Written for commit 310c3f0. Summary will update on new commits.}

[Ignacio-Vidal]

@Mattias-Sehlstedt - This is an enhancement to your recent contribution 439f608 (introduce DiscriminatorUtils). Could you review it please?

@wing328 - Could you review i? Given you approved 439f608 too

[cubic-dev-ai]

1 issue found and verified against the latest diff

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[Mattias-Sehlstedt]

Hi, see #24158 and #24156 for similar changes. For your change I would say that the same as in #24156 (comment) applies (i.e., is it really necessary to do two recursive passes?).