DoubaoStreamCompleted(图像生成完成事件)

所有图片处理完毕后推送的最终事件，包含使用统计信息

{
    "type": "image_generation.completed",
    "model": "doubao-seedream-4-0-250828",
    "created": 1589478378,
    "usage": {
        "generated_images": 2,
        "output_tokens": 4096,
        "total_tokens": 4096
    }
}