[BUG] [Java][Spring] Model composition does not work for string type

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?
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Description

It is sometimes useful to have a base type which is derived from string with all constraints, refer this type using "ref" syntax and change just the description. The problem is that generation does not work correctly for the case shown below. @Size annotation will be not generated as expected.

openapi-generator version

I use 5.0.0-beta2 in my test

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: test
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: 'https://www.apache.org/licenses/LICENSE-2.0.html'
paths:
  /pet:
    post:
      description: ''
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    Pet:
      title: a Pet
      description: A pet for sale in the pet store
      type: object
      properties:
        test1:
          allOf:
            - $ref: '#/components/schemas/StringMinMax'
            - description: "Property one description"
        test2:
          allOf:
            - $ref: '#/components/schemas/StringMinMax'
            - description: "Property two description"
    StringMinMax:
      type: string 
      minLength: 1
      maxLength: 5

Generation Details

java -jar openapi-generator-cli-5.0.0-beta2.jar generate -i petstore.yaml -g spring

Steps to reproduce

1. Generate classes
2. Look inside Pet class. Size annotations are missing on getters

Related issues/PRs

Suggest a fix

Start by running the provided openapi-generator command with the YAML declaration and inspect the generated Pet class getters. Trace how the Spring generator handles the allOf composition for StringMinMax. Done means the generated getters include the expected @Size constraints for both test1 and test2.