[BUG][JAVA] allOf with just one element generates incorrect validateJsonElement

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

When I provide only one element in allOf, then autogenerated java code doesn't compile, because it calls validateJsonElement on Object class.

openapi-generator version

7.9.0

OpenAPI declaration file content or url

Below yaml with only one element in allOf:

openapi: 3.1.0
info:
  title: Product API
  version: 1.0.0
paths:
  /product:
    get:
      summary: Get a product
      operationId: getProduct
      responses:
        '200':
          description: Successful response
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Product'

components:
  schemas:
    Product:
      type: object
      title: Product
      required:
        - product
      properties:
        product:
          $ref: '#/components/schemas/productDetails'
      additionalProperties: false

    productDetails:
      type: object
      allOf:
        - required:
            - productName
      properties:
        productName:
          type: string

generates such code

      // check to make sure all required properties/fields are present in the JSON string
      for (String requiredField : Product.openapiRequiredFields) {
        if (jsonElement.getAsJsonObject().get(requiredField) == null) {
          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonElement.toString()));
        }
      }
        JsonObject jsonObj = jsonElement.getAsJsonObject();
      // validate the required field `product`
      Object.validateJsonElement(jsonObj.get("product"));
     //^Cannot resolve method 'validateJsonElement' in 'Object' <-- error I see form above line in my IDE
  }

but when I add second element to allOf, for example:

      allOf:
        - required:
            - productName
        - format: uri

it works fine and java code looks like this:

      // check to make sure all required properties/fields are present in the JSON string
      for (String requiredField : Product.openapiRequiredFields) {
        if (jsonElement.getAsJsonObject().get(requiredField) == null) {
          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonElement.toString()));
        }
      }
        JsonObject jsonObj = jsonElement.getAsJsonObject();
      // validate the required field `product`
      ProductDetails.validateJsonElement(jsonObj.get("product"));

Generation Details

I use java and gradle and this is my gradle task to generate docs:

tasks.named("openApiGenerate", org.openapitools.generator.gradle.plugin.tasks.GenerateTask::class) {
    generatorName.set("java")
    inputSpec.set("$rootDir/openapi/bundled.yaml")
    outputDir.set("$buildDir/generated")
    apiPackage.set("com.yourcompany.api")
    modelPackage.set("com.yourcompany.model")
    invokerPackage.set("com.yourcompany.invoker")
    configOptions.set(mapOf(
        "dateLibrary" to "java8",
        "java8" to "true"
    ))
}

Steps to reproduce

just run openApiGenerate with provided openapi.
Expected behaviour is that generated code compiles and work fine even if there is only one element in allOf.

Related issues/PRs

Suggest a fix

Start with the Java generator invoked by the openApiGenerate Gradle task and reproduce the issue using the minimal OpenAPI YAML in the report. Compare generated validation for a one-element allOf with the two-element case; done means the generated Java code references the model type correctly and compiles.