Support nullable referenced schemas using anyOf with type: null

[zunda-pixel]

Description

When an OpenAPI 3.1 schema uses anyOf with a $ref and a type: "null" branch, Swift OpenAPI Generator does not appear to generate the expected optional Swift property.

Minimal schema:

openapi: 3.1.0
info:
  title: Repro
  version: 1.0.0
paths: {}
components:
  schemas:
    Parent:
      type: object
      properties:
        child:
          anyOf:
            - $ref: '#/components/schemas/Child'
            - type: 'null'
      required:
        - child
    Child:
      type: object
      properties:
        name:
          type: string
      required:
        - name

Expected behavior

The generated Swift type for Parent should allow child to be decoded/encoded as a nullable referenced object, for example as an optional Child-like generated property.

Actual behavior

The generator did not produce a usable optional property for this schema shape in my project. I had to switch to this workaround:

child:
  allOf:
    - $ref: '#/components/schemas/Child'
  nullable: true

That workaround generates a usable optional Swift property, but it also emits warnings because nullable is not part of the OpenAPI 3.1 schema vocabulary:

Found 'nullable' property. This property is not supported by OpenAPI v3.1.0.
OpenAPIKit has translated it into 'type: ["null", ...]'.

Why this matters

OpenAPI 3.1 supports JSON Schema's null type, so anyOf/oneOf with a type: "null" branch is a common way to express a nullable $ref. It would be useful if Swift OpenAPI Generator supported this directly, or documented the recommended schema shape for nullable referenced objects.

Environment

• Swift OpenAPI Generator: 1.12.0
• OpenAPI document version: 3.1.0
• Swift toolchain: Xcode toolchain, Swift 6.x