[BUG][Kotlin] openapi 3.0.3 allOf + description + nullable = intermediate class created

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

7.0 had many improvements to avoid intermediate classes. However, there is a case where no intermediate class was created in 6.4.x but is in 7.4.0

openapi-generator version

openapi-generator-maven-plugin 7.2.0
generatorName: kotlin-spring
useSpringBoot3: true

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Swagger Petstore - OpenAPI 3.0
  version: 1.0.11
servers:
  - url: "/api"
paths:
  /user/logout:
    get:
      summary: Logs out current logged in user session
      description: ''
      operationId: logoutUser
      parameters: []
      responses:
        default:
          description: successful operation
      security:
        - api_key: []
components:
  schemas:
    A:
      type: object
      allOf:
        - $ref: '#/components/schemas/B'
        - type: object
          properties:
            propWithMissingDescription:
              allOf:
                - $ref: '#/components/schemas/SomeRef'
              description: "this comment doesn't show up on the prop (but on the intermediate class)"
              nullable: true # <-- makes that an intermediate class is created
            workaround:
              allOf:
                - type: object
                  allOf:
                    - $ref: '#/components/schemas/SomeRef'
                  description: "this comment shows up"
              nullable: true # <-- works 
    B:
      type: object
      allOf:
        - type: object
          properties:
            name:
              type: string
    SomeRef:
      type: object
      properties:
        a:
          type: string
        b:
          type: string
  securitySchemes:
    api_key:
      type: apiKey
      name: api_key
      in: header       

Generation Details

Steps to reproduce

Related issues/PRs

similar to https://github.com/OpenAPITools/openapi-generator/issues/17726

Suggest a fix

Start with the kotlin-spring generator and the supplied OpenAPI 3.0.3 declaration, reproducing the generated models for the propWithMissingDescription and workaround schemas. Compare the output to confirm whether nullable creates an intermediate class and whether the property description is retained; done means the problematic property no longer produces the unintended intermediate class while preserving its description.