transcription response should also have `text/plain` content type

the transcription response should also have text/plain content type for art, vtt, and text response_formats.

the last 3 lines should be added.

  /audio/transcriptions:
    post:
      operationId: createTranscription
      tags:
        - Audio
      summary: Transcribes audio into the input language.
      requestBody:
        required: true
        content:
          multipart/form-data:
            schema:
              $ref: "#/components/schemas/CreateTranscriptionRequest"
      responses:
        "200":
          description: OK
          content:
            application/json:
              schema:
                oneOf:
                  - $ref: "#/components/schemas/CreateTranscriptionResponseVerboseJson"
                  - $ref: "#/components/schemas/CreateTranscriptionResponseJson"
            text/plain:
              schema:
                type: string

Locate the OpenAPI definition for the /audio/transcriptions POST endpoint and review its 200 response content. Add the text/plain string response for the art, vtt, and text formats, then validate the specification or inspect the diff to confirm the response content is represented correctly.