ChatCompletionChunk

One SSE data: payload. Text arrives in choices[].delta.content; tool calls in
choices[].delta.tool_calls (accumulate by index). The final choice chunk includes
finish_reason. With stream_options.include_usage a usage-only chunk is added. In Chat
Completions streaming, debug appears on the final chunk with a finish reason.

{
    "id": "string",
    "object": "chat.completion.chunk",
    "choices": [
        {
            "index": 0,
            "delta": {
                "content": "string",
                "tool_calls": [
                    {
                        "property1": "string",
                        "property2": "string"
                    }
                ]
            },
            "finish_reason": "string"
        }
    ],
    "usage": {
        "prompt_tokens": 0,
        "completion_tokens": 0,
        "total_tokens": 0,
        "prompt_tokens_details": {
            "cached_tokens": 0
        },
        "completion_tokens_details": {
            "reasoning_tokens": 0
        }
    },
    "debug": {
        "normalizations": [
            {
                "param": "string",
                "action": "add",
                "before": null,
                "after": null
            }
        ],
        "deprecations": [
            {
                "param": "string",
                "message": "string"
            }
        ],
        "routing": [
            {
                "provider": "string",
                "region": "string",
                "status": "failed",
                "target": "string"
            }
        ]
    }
}