[BUG] [scala-sttp] Generated code fails to compile when multipart/form-data includes the required fields

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

Generated code fails to compile when the requestbody of multipart/form-data includes the required field(s).

openapi-generator version

The version 6.x.x and the latest master branch (6.3.0).

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: multipart requestBody
  version: '1.0'
paths:
  /translations/upload:
    post:
      operationId: translations_upload
      parameters:
      - description: Client Instance Uuid.
        explode: false
        in: path
        name: client_instance_uuid
        required: true
        schema:
          format: uuid
          type: string
        style: simple
      requestBody:
        content:
          multipart/form-data:
            encoding:
              translation_file:
                style: form
            schema:
              $ref: '#/components/schemas/translations_upload_request'
      responses:
        "200":
          content:
            application/json:
              schema:
                format: binary
                type: string
      summary: Upload translations
      x-content-type: multipart/form-data
      x-accepts: application/json
components:
  schemas:
    translations_upload_request:
      properties:
        translation_file:
          description: Gets or sets translation file (json/zip).
          format: binary
          type: string
      required:
      - translation_file
      type: object

Generation Details

Command line used for generation:

openapi-generator-cli generate \
  -g scala-sttp \
  -i name_of_file.yaml \
  -p mainPackage=myPackage \
  -o myApi

Issues in the generated client:

No implicit view available from sttp.model.Part[sttp.client3.RequestBody[Any]] => scala.collection.IterableOnce[B].

  def translationsUpload(clientInstanceUuid: UUID, translationFile: File
): Request[Either[ResponseException[String, Exception], File], Any] =
    basicRequest
      .method(Method.POST, uri"$baseUrl/translations/upload")
      .contentType("multipart/form-data")
      .multipartBody(Seq(
                multipartFile("translation_file", translationFile)
      ).flatten)            <---- compile error
      .response(asJson[File])
}

Related issues/PRs

Suggest a fix

The issue seems in https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/scala-sttp/paramMultipartCreation.mustache#L1-L8

can someone review the template above?

Start with modules/openapi-generator/src/main/resources/scala-sttp/paramMultipartCreation.mustache and reproduce the issue using the provided openapi-generator-cli command and YAML specification. Inspect the generated Scala sttp client around multipartBody and verify that the generated code compiles when the multipart/form-data field is required.