[BUG] [Spring] OneOf base class 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)?
• 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

While generating a Java server code with polymorphism (oneOf), we get a compilation error while the base class is imported due to it was not generated.

openapi-generator version

5.2.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: OneOf example
  version: '1.0.0'
paths:
  '/vegetables':
    get:
      summary: Get vegetables
      responses:
        '200':
          description: OK
          content:
            application/json:
              schema:
                oneOf:
                - $ref: '#/components/schemas/Tomato'
                - $ref: '#/components/schemas/Carrot'
        '404':
          description: Not Found
      operationId: getVegetables
components:
  schemas:
    Tomato:
      type: object
      properties:
        property1:
          type: integer
        property2:
          type: string
    Carrot:
      type: object
      properties:
        property3:
          type: integer
        property4:
          type: string  

Plugin configuration:

<plugin>
	<version>5.2.0</version>
    <groupId>org.openapitools</groupId>
    <artifactId>openapi-generator-maven-plugin</artifactId>
    <executions>
        <execution>
            <id>generate-rest-api</id>
            <goals>
                <goal>generate</goal>
            </goals>
            <phase>generate-sources</phase>
            <configuration>
                <inputSpec>${project.basedir}/src/main/resources/openapi-rest.yml</inputSpec>
                <generatorName>spring</generatorName>
                <generateSupportingFiles>true</generateSupportingFiles>                
                <configOptions>
                    <dateLibrary>java8</dateLibrary>
                    <java8>true</java8>
                    <interfaceOnly>true</interfaceOnly>
                    <apiPackage>com.example</apiPackage>                    
                    <modelPackage>com.example.dto</modelPackage>
                    <sourceFolder>src/gen/java/main</sourceFolder>                    
                </configOptions>
            </configuration>
        </execution>       
    </executions>    
</plugin>

Generation Details

Steps to reproduce

Generate code with openapi-generator-maven-plugin.

• Add the openapi-generator-maven-plugin to a pom.xml
• Add the previous OpenAPI to a file named openapi-rest.yml at ${project.basedir}/src/main/resources/
• Execute mvn clean compile
• Check the compilation error at DefaultApi.java file. It's trying to import a OneOfTomatoCarrot, but it has not been generated.

Related issues/PRs

• https://github.com/OpenAPITools/openapi-generator/pull/5661
• https://github.com/OpenAPITools/openapi-generator/issues/5381

Suggest a fix

Following the provided example, a OneOfTomatoCarrot class must be generated.

Reproduce the issue with the provided OpenAPI declaration and Spring generator configuration, then inspect the generated DefaultApi.java import and the missing OneOfTomatoCarrot class. Trace the oneOf model generation path and verify that the base class is generated and the resulting project compiles with mvn clean compile.