MCP Server 'Command to run' strips backslashes from Windows paths

### Describe the bug

When adding an MCP server in Copilot CLI, the **Command to run** field incorrectly processes Windows-style paths containing backslashes (`\`). The backslashes are stripped, causing the saved command path to become invalid.

This is a common scenario when pointing Copilot CLI at a locally built MCP server executable.

### Steps to reproduce

1. Open Copilot CLI settings/config UI for **MCP Servers**
2. Add a new MCP server (or edit an existing one)
3. In **Command to run**, enter a Windows path with backslashes, e.g.:
`C:\src\external\gitrepo\mcpserver\bin\mcpserver.exe`
4. Save

### Actual behavior

The saved value strips the backslashes, e.g. it becomes:
`C:srcexternalgitrepomcpserverbinmcpserver.exe`

### Expected behavior

The saved command should preserve the Windows path exactly as entered (including backslashes), e.g.:
`C:\src\external\gitrepo\mcpserver\bin\mcpserver.exe`

### Environment

- OS: Windows
- Scenario: locally built MCP server executable path entered into “Command to run”

### Additional context / notes

This looks like an escaping/serialization issue (e.g., treating backslashes as escape characters) when persisting the MCP server config. The UI should either:
- properly escape backslashes when storing, or
- treat the field as a raw string, or
- clearly document/accept `\\` if that’s required (but ideally it shouldn’t be).

在 Windows 上使用报告中的路径，在 MCP Servers 配置 UI 中重现该问题，然后跟踪 Command to run 值的持久化方式。当保存并重新打开 MCP server 后，输入的 Windows 路径中的每个反斜杠都能保留时，即表示完成。

由索引模型根据 Issue 内容生成。