[BUG][java] oneOf in "anonymous" schemas: model not generated.

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• What's the version of OpenAPI Generator used?
• Have you search for related issues/PRs?
• What's the actual output vs expected output?
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Description

As mentioned in #5706, if the openapi spec file defines a oneOf schema in responses "as is", the generator references a model in the DefaultApi but the model doesn't get generated. See examples below.
edit: the same issue happens to all the places where oneOf is used without being directly under a named schema (e.g. responses, array items etc.). See the comments.

openapi-generator version

latest master: cc09118ffc2f13ca5d251049dc3d7552f08b670a

OpenAPI declaration file content or url

OpenApi spec file:

openapi: 3.0.3
info:
  title: oneOf test
  version: '1.0'
paths:
  /state:
    get:
      operationId: getState
      responses:
        '200':
          description: get states
          content:
            application/json:
              schema: # doesn't work
                oneOf:
                  - $ref: '#/components/schemas/ObjA'
                  - $ref: '#/components/schemas/ObjB'
                discriminator:
                  propertyName: realtype
  /state_ref:
    get:
      operationId: getStateRef
      responses:
        '200':
          description: 'get states using $ref'
          content:
            application/json:
              schema: # logically same as above but works
                $ref: '#/components/schemas/ObjAOrObjB'
components:
  schemas:
    ObjA:
      type: object
      properties:
        realtype:
          type: string
        message:
          type: string
    ObjB:
      type: object
      properties:
        realtype:
          type: string
        code:
          type: integer
    ObjAOrObjB:
      oneOf:
        - $ref: '#/components/schemas/ObjA'
        - $ref: '#/components/schemas/ObjB'
      discriminator:
        propertyName: realtype

Command line used for generation

openapi-generator generate -i spec.yaml -g java --library native -o ./out/java_oneof

Steps to reproduce

• use spec file above
• run command above
• notice how src/main/java/org/openapitools/client/api/DefaultApi.java features

  import org.openapitools.client.model.GetState200;
  import org.openapitools.client.model.ObjAOrObjB;
  

  but the GetState200 model is not present in src/main/java/org/openapitools/client/model while ObjAOrObjB is.

@bbdouglas @sreeshas @jfiala @lukoyanov @cbornet @jeff9finger @karismann @Zomzog @lwlee2608 @bkabrda

Run the supplied OpenAPI YAML with openapi-generator generate -i spec.yaml -g java --library native -o ./out/java_oneof. Compare src/main/java/org/openapitools/client/api/DefaultApi.java with src/main/java/org/openapitools/client/model, focusing on the imported GetState200 and the generated ObjAOrObjB; done means the referenced response model is present and the generated client is consistent.