[BUG] ERROR! Not handling <code> as Body Parameter at the moment

Description

I get the following error when trying to generate a Go client:

[main] ERROR o.o.codegen.DefaultCodegen - ERROR! Not handling  class Parameter {
    name: null
    in: null
    description: null
    required: null
    deprecated: null
    allowEmptyValue: null
    style: null
    explode: null
    allowReserved: null
    schema: null
    examples: null
    example: null
    content: null
    $ref: #/components/parameters/idempotencyKey
} as Body Parameter at the moment

This comes about when my root.yaml file contains a paths sub-object with a $ref to an external OpenAPI spec file for a path like:

paths:
  /originators:
    post:
      tags:
        - Originators
      summary: Create a new Originator object
      operationId: addOriginator
      security:
        - bearerAuth: []
        - cookieAuth: []
      parameters:
        - $ref: '#/components/parameters/requestId'
        - $ref: '#/components/parameters/idempotencyKey'
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/CreateOriginator'        

  parameters:
    idempotencyKey:
      in: header
      name: X-Idempotency-Key
      description: Idempotent key in the header which expires after 24 hours. These strings should contain enough entropy for to not collide with each other in your requests.
      example: a4f88150
      required: false
      schema:
        type: string

openapi-generator version

4.0.0-beta2

Steps to reproduce

OPENAPI_GENERATOR_VERSION=4.0.0-beta2 ./openapi-generator generate -i openapi.yaml -g go -o ./client

Suggest a fix

Looking at the relevant source code this path is simply commented out (and has been for quite some time), so I assume there's a refactor needed?

I'm willing to offer a patch which fixes this, but I'm unsure of the history or what all would need to be refactored. Any advice? I'll contribute a reproducible local test as part of the PR.

Start in modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java at the referenced parameter-handling path, then reproduce the failure with the supplied openapi-generator command and YAML. Add the mentioned local regression test and verify that Go client generation completes without the “Not handling ... as Body Parameter” error.