[BUG][JAVA][RETROFIT2] Generates unusable class for anyOf

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?

Description

The java generator/retrofit2 library does not appear to support anyOf. It generates a valid but empty Java class, which is missing all infrastructure for the specified types.

openapi-generator version

Version 7.9.0

OpenAPI declaration file content or url

openapi: 3.0.3

info:
  title: Test
  version: 1

paths:
  /forms:
    get:
      responses:
        '200':
          description: 'Test'
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Processor'

components:
  schemas:
    KnownProcessorName:
      type: string
      enum: ['PerformPostponedActionService']

    Processor:
      type: object
      properties:
        name:
          $ref: '#/components/schemas/ProcessorName'
        timestampProcessed:
          type: string
          format: date-time

    ProcessorName:
      anyOf:
        - type: string
        - $ref: '#/components/schemas/KnownProcessorName'

Generation Details

docker run --rm -v "${pwd}:/local" openapitools/openapi-generator-cli generate -i /local/test.yaml -g java -o /local/out/java --library=retrofit2 --additional-properties=annotationLibrary=none,openapiNullable=false 

Steps to reproduce

• Save the provided yaml as test.yaml
• Run the specified docker command
• Observe the ProcessorName class. It compiles, but only contains some generic Java basics, no way to specify either a string or a KnownProcessorName

Start with the provided test.yaml and run the Docker generation command for the java generator with the retrofit2 library. Inspect the generated ProcessorName class and how the anyOf schema is handled. Done means the generated class provides usable infrastructure for either a string or KnownProcessorName, rather than an effectively empty class.