多轮对话（流式）

{
    "contents": [
        {
            "role": "user",
            "parts": [
                {
                    "text": "Hello"
                }
            ]
        },
        {
            "role": "model",
            "parts": [
                {
                    "text": "Great to meet you. What would you like to know?"
                }
            ]
        },
        {
            "role": "user",
            "parts": [
                {
                    "text": "I have two dogs in my house. How many paws are in my house?"
                }
            ]
        }
    ]
}