[BUG] [JAVA] openapi-generator-maven-plugin does not handle allOf composed of multiple objects

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

Using allOf composed of multiple objects in a response does not generate a class combining these multiple objects. Instead it uses the first class defined in the allOf section.

openapi-generator version

Version 5.2.0:

<plugin>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <version>5.2.0</version>
</plugin>

I have also tested with master (5.2.1-SNAPSHOT), commit fe9636e9.

OpenAPI declaration file content or url

Important part here is the allOf:

openapi: 3.0.0
info:
  title: Test
  version: v1
paths:
  /test:
    get:
      operationId: get
      responses:
        '200':
          description: Get
          content:
            application/json:
              schema:
                allOf:
                  - $ref: '#/components/schemas/A'
                  - $ref: '#/components/schemas/B'
components:
  schemas:
    A:
      type: object
      properties:
        attA:
          type: string
    B:
      type: object
      properties:
        attB:
          type: integer

Generation Details

generatorName: java
inputSpec: openapi.yaml

Steps to reproduce

 mvn org.openapitools:openapi-generator-maven-plugin:5.2.0:generate \
    -Dopenapi.generator.maven.plugin.inputSpec=openapi.yaml \
    -Dopenapi.generator.maven.plugin.generatorName=java

Shows a warning:

[WARNING] allOf with multiple schemas defined. Using only the first one: A

It generates classes A and B. But when calling get(), the value returned by the call is A:

DefaultApi api = new DefaultApi();
A a = api.get();

I expected instead a composite object with A and B properties (attA and attB), like this (result from https://editor.swagger.io/):

Related issues/PRs

Suggest a fix

Start by running the Maven plugin command with the supplied openapi.yaml and inspect the generated DefaultApi return type and the A and B classes. Trace the handling of the response schema's multiple allOf references from the Maven generation entry point. Done means the generated result combines attA and attB rather than selecting only A, and the warning is no longer produced.