allOf that narrows an enum fails with "Cannot take allOf a non-object" #1498

If a property narrows an enum with allOf (a $ref to the enum plus a shorter enum list), the generator drops the model, along with every model that references it.

components:
  schemas:
    CloudProvider:
      type: string
      enum: [AWS, AZURE]
    Region:
      type: object
      required: [provider]
      properties:
        provider:
          allOf:
            - $ref: '#/components/schemas/CloudProvider'
            - enum: [AWS]

On 0.29.1 this gives Cannot take allOf a non-object and Region is dropped. In our spec, a handful of these ended up dropping models used by 20 endpoints.

I'd expect provider to come out as CloudProvider, ideally limited to AWS. merge_properties already merges two enums like this when they come from an object-level allOf. It just never gets called when the allOf sits on the property itself, because that case goes straight to ModelProperty.build.

I opened a possible fix in #1499. Same caveat as there: an AI agent wrote it and it has no tests yet.