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_

透過呼叫 StructuredContentBigIntMcp 專案的 return_big_numbers 工具，重現儲存庫 .mcp.json 設定中的失敗。追蹤結構化 MCP 回應在哪裡進行序列化，並確認 BigInt 值的處理方式。當回應不再引發序列化錯誤或中止進行中的工作時，即表示完成。

由索引模型根據 Issue 內容生成。