[BUG][JAVA] UNKNOWN_BASE_TYPE when using $ref to model in requestBodies

Created by: Swiftwork

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
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Description

Openapi generator fails to handle $ref to model in requestBodies resulting in UNKOWN_BASE_TYPE.

openapi-generator version

Version: 5.4.0 Might be a regression as this has been mentioned before: https://github.com/OpenAPITools/openapi-generator/issues/2575

OpenAPI declaration file content or url

openapi: 3.0.0
paths:
  /user:
    post:
      summary: Create New User
      operationId: post-user
      requestBody:
        $ref: '#/components/requestBodies/UserPost'
      responses:
        '200':
          description: OK
components:
  requestBodies:
    UserPost:
      content:
        application/json:
          schema:
            type: object
            properties:
              firstName:
                type: string
              lastName:
                type: string

Generation Details

Using typescript-fetch

openapi-generator-cli generate -g typescript-fetch -i spec.yaml -o gen

Steps to reproduce

Reproduction

Related issues/PRs

https://github.com/OpenAPITools/openapi-generator/issues/2575