chart组件toolbox自定义事件无法绑定函数

描述问题：

在使用chart组件时，自定义toolbox的feature无法绑定事件。

截图或视频：

如何复现（请务必完整填写下面内容）：

1. 你是如何使用 amis 的？
   sdk或npm

2. amis 版本是什么？请先在最新 beta 版本测试问题是否存在
   3.1

3. 粘贴有问题的完整 amis schema 代码：

{
  "type": "page",
  "body": {
    "type": "chart",
    "api": "/amis/api/mock2/chart/chartData",
    "config": {
      "toolbox": {
        "show": true,
        "feature": {
          "myDownload": {
            "show": true,
            "title": "数据导出",
            "icon": "image://https://echarts.apache.org/zh/images/favicon.png",
            "onclick": "function () { alert(\"need help\");}"
          }
        }
      },
      "xAxis": {
        "type": "category",
        "data": [
          "Mon",
          "Tue",
          "Wed",
          "Thu",
          "Fri",
          "Sat"
        ]
      },
      "yAxis": {
        "type": "value"
      },
      "series": [
        {
          "data": "${line || []}",
          "type": "line"
        }
      ]
    }
  }
}

4. 操作步骤
   无需其他操作，直接就会报错

Start by reproducing the issue with the provided amis chart schema and its custom toolbox feature, then trace the chart component's handling of the toolbox configuration and onclick value. Done means the myDownload feature binds its custom handler without an error and triggers the expected alert when selected.