【bug】kit 合并消息流部分场景存在 bug

[chilingling]

【问题描述】：
部分场景下，大模型流式可能会同时返回 delta 和 message，这个时候需要兼容处理。
示例：魔搭 modelscope 会同时返回 deleta 和空 message。

{
    "id": "chatcmpl-052745f8-e891-9566-bbf5-8b56dc1b8478",
    "object": "chat.completion.chunk",
    "created": 1769842040,
    "model": "Qwen/Qwen3-VL-235B-A22B-Instruct",
    "system_fingerprint": "",
    "choices": [
        {
            "index": 0,
            "delta": {
                "role": null,
                "content": "",
                "tool_calls": null,
                "function_calls": null,
                "reasoning_content": ""
            },
            "finish_reason": "stop",
            "message": {
                "role": null,
                "content": "",
                "tool_calls": null,
                "function_calls": null,
                "reasoning_content": ""
            }
        }
    ],
    "usage": {
        "prompt_tokens": 0,
        "completion_tokens": 0,
        "total_tokens": 0
    }
}

关联代码：

tiny-robot/packages/kit/src/vue/message/useMessage.ts

Lines 199 to 222 in b599aa5

	if (choice) {
	lastChoice = choice
	const runDefault = () => {
	// Ensure metadata exists
	if (!message.metadata) {
	message.metadata = {}
	}
	const { created, ...rest } = chunk
	message.metadata.createdAt = created
	message.metadata.updatedAt = Math.floor(Date.now() / 1000)
	Object.assign(message.metadata, rest)
	combileDeltaData(message, choice.message || choice.delta)
	}
	if (onCompletionChunk) {
	const baseContext = getBaseContext(abortSignal)
	onCompletionChunk({ ...baseContext, chunk, choice, currentMessage: message }, runDefault)
	} else {
	runDefault()
	}
	}