Copilot CLI couldn't handle BigInt in structured MCP response

### Describe the bug

When a MCP server returns big numbers in its response, Copilot CLI cannot handle it and fails with the following error. All ongoing tasks will be aborted
```
Execution failed: TypeError: Do not know how to serialize a BigInt
```

### Affected version

1.0.74-0

### Steps to reproduce the behavior

Requires dotnet 10.
1. Clone the repo https://github.com/xj-ms/StructuredContentBigIntMcp
2. Launch Copilot CLI in the repo. The MCP server should be already configured with the local .mcp.json
3. Ask Copilot to call the `return_big_numbers` tool
4. Observe the failure


### Expected behavior

_No response_

### Additional context

_No response_

通过调用 StructuredContentBigIntMcp 项目的 return_big_numbers 工具，复现仓库 .mcp.json 配置中的故障。跟踪结构化 MCP 响应在哪里被序列化，并确定 BigInt 值的处理方式。当响应不再引发序列化错误或中止正在进行的任务时，即视为完成。

由索引模型根据 Issue 内容生成。