[BUG][JAVA] Invalid code generated for java/apache-httpcomponent when using polymorphism

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• 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?
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Description

I'm trying to generate apache-httpclient client from a spec where Store object has a reference to either Pet or Cat which inherit from Pet

The generated code for StorePet.java contains following code in the public String toUrlQueryString(String prefix) method:

    // add `huntingSkill` to the URL query string
    if (getHuntingSkill() != null) {
      try {
        joiner.add(String.format("%shuntingSkill%s=%s", prefix, suffix, URLEncoder.encode(String.valueOf(getHuntingSkill()), "UTF-8").replaceAll("\\+", "%20")));
      } catch (UnsupportedEncodingException e) {
        // Should never happen, UTF-8 is always supported
        throw new RuntimeException(e);
      }
    }

However the method getHuntingSkill() (or the property huntingSkill) is not present in the class.

openapi-generator version

7.15.0. Not a regression, observed in 6.x versions as well.

OpenAPI declaration file content or url

components:
  schemas:
    Store:
      type: object
      properties:
        pet:
          oneOf:
            - $ref: "#/components/schemas/Cat"
            - $ref: "#/components/schemas/Dog"
          discriminator:
            propertyName: petType
            mapping:
                Cat:  "#/components/schemas/Cat"
                Dog:  "#/components/schemas/Dog"
    Pet:
      discriminator:
        propertyName: petType
      required:
        - name
        - petType # required for inheritance to work
      properties:
        name:
          type: string
        petType:
          type: string
    Cat:
      allOf:
        - $ref: '#/components/schemas/Pet' # Cat has all properties of a Pet
        - properties: # extra properties only for cats
            huntingSkill:
              type: string
              default: lazy
              enum:
                - lazy
                - aggressive
    Dog:
      allOf:
        - $ref: '#/components/schemas/Pet' # Dog has all properties of a Pet
        - properties: # extra properties only for dogs
            packSize:
              description: The size of the pack the dog is from
              type: integer
openapi: 3.0.3
security: []
servers: []
paths: {}
info:
  version: 1.0.0
  title: Swagger Petstore

Generation Details

language: java
library: apache-httpclient

Steps to reproduce

openapi-generator generate -i src/main/resources/test.store.openapi.yaml -g java -o tmp/

Related issues/PRs

Start by running the listed openapi-generator command with src/main/resources/test.store.openapi.yaml and inspect the generated StorePet.java. Trace how the Store, Pet, Cat, and Dog polymorphism is represented in the generated model and its toUrlQueryString method. Done means the apache-httpclient Java output does not reference properties absent from the generated class.