[BUG][JAVA] Multipart Content-ID header missing from client library generation

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?
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Description

given a multipart endpoint description like this:

requestBody:
  content:
    multipart/form-data:
      schema:
        type: object
        properties:
          json_metadata:
            $ref: '#/components/schemas/myschema'
          files:
            type: array
            items:
              type: file
              format: binary
      encoding:
        json_metadata:
          contentType: application/json
        files:
          contentType: application/octet-stream
          headers:
            Content-Id:
              schema:
                type: string

When Generating a client in java (or any language/generator that I tried, for that matter), I do not see any reference to the Content-Id header. it does not appear to be set or available to set. Am I missing something?

openapi-generator version

6.6.0

OpenAPI declaration file content or url

https://www.3gpp.org/ftp/Specs/archive/OpenAPI/Rel-15/TS29518_Namf_Communication.yaml

Generation Details

java -jar openapi-generator-cli-6.6.0.jar generate -i ./my-api.yaml -g java --global-property skipFormModel=false

Steps to reproduce

specify a Content-Id header in a multipart request. generate a client. there will be no reference to Content-Id in any of the generated code.

Related issues/PRs

Suggest a fix

Start with the linked TS29518_Namf_Communication.yaml and reproduce generation using openapi-generator-cli 6.6.0 with the Java generator and skipFormModel=false. Trace how the multipart encoding and its Content-Id header are represented in the generated client; done means the generated code includes a way to set or emit that header.