[BUG][JAVA] Java imports for allOf schema in parameter broken in 6.0.0

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

With version 6.0.0 there came a change in handling imports for operation parameter types in generated API classes. If there is an parameter with schema that uses allOf, the "parent" class will be incorrectly imported instead of the actual class.

Version 5.4.0:

package org.openapitools.api;

...
import org.openapitools.model.BuggedParameterType;
...

@Client("${base-path}")
public interface BugApi {

    @Get(uri="/test/path")
    @Consumes(value={"application/json"})
    Mono<Object> buggedOperation(
        @QueryValue(value="buggedParameter") @NotNull @Valid BuggedParameterType buggedParameter
  );
}

Version 6.0.0:

package org.openapitools.api;

...
import org.openapitools.model.BuggedParameterBaseType;
...

@Client("${base-path}")
public interface BugApi {

    @Get(uri="/test/path")
    @Consumes(value={"application/json"})
    Mono<Object> buggedOperation(
        @QueryValue(value="buggedParameter") @NotNull @Valid BuggedParameterType buggedParameter
  );
}

This leads to compile error, since the actual parameter type is not imported.

This example is Java Micronaut output, but I'm quite sure this is applies to generic Java output as well.

I created a repository for easier issue reproduction here: https://github.com/oujesky/openapi-generator-java-import-bug-repro-steps

openapi-generator version

5.4.0 - OK
6.0.0 - incorrect

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Bug report
  version: '1.0'
paths:
  /test/path:
    get:
      tags:
        - Bug
      operationId: buggedOperation
      parameters:
        - name: buggedParameter
          in: query
          required: true
          explode: true
          schema:
            $ref: '#/components/schemas/BuggedParameterType'
      responses:
        200:
          description: OK
components:
  schemas:
    BuggedParameterBaseType:
      type: object
      properties:
        x:
          type: string
        y:
          type: string
    BuggedParameterType:
      allOf:
        - $ref: '#/components/schemas/BuggedParameterBaseType'
        - type: object
          properties:
            z:
              type: string

Generation Details

Default settings for java-micronaut-client (but should be reproducible for other outputs as well)

openApiGenerate {
    inputSpec = "$rootDir/openapi.yml"
    generatorName = 'java-micronaut-client'
    outputDir = "$buildDir/openapi"
}

Steps to reproduce

• clone https://github.com/oujesky/openapi-generator-java-import-bug-repro-steps
• run ./gradlew openApiGenerate
• check openapi-generator-5.4.0/build/openapi/src/main/java/org/openapitools/api/BugApi.java - imports are OK
• check openapi-generator-6.0.0/build/openapi/src/main/java/org/openapitools/api/BugApi.java - imports are incorrect

Related issues/PRs

• #12369
• #12724

Suggest a fix

I was able to somewhat pinpoint the place where this is happening to changes coming from #12369 - specifically changes in DefaultCodegen#fromParameter(...) method (https://github.com/OpenAPITools/openapi-generator/pull/12369/files#diff-3a138675e8cb40943bcb00feb46edd6f6ee5c5306c29cc19f02c845a299c8658L4580) that incorrectly returns information about the "parent" class, instead of the actual class that is then added to imports.

Unfortunately I'm not proficient with the code enough to suggest an actual fix.

Start with DefaultCodegen#fromParameter and compare the import-related changes associated with #12369. Reproduce the issue using the linked repository and ./gradlew openApiGenerate, then compare the generated BugApi.java files for versions 5.4.0 and 6.0.0. Done means the generated Java API imports BuggedParameterType rather than BuggedParameterBaseType and no longer has the reported compile error.