Terminal failed to start with "Access is denied" error

### Short summary

Terminal initialization fails with access denied error when attempting to spawn PowerShell process.

### Affected version or release

v1.1.8

### Installation context

GitHub Copilot installed on Windows, attempting to spawn terminal with shell integration

### What happened?

When opening a terminal session, the application fails to spawn the PowerShell process with the following error:

```
Failed to spawn PTY command: CreateProcessW `"C:\\Users\\hnaganur\\AppData\\Local\\Microsoft\\WindowsApps\\pwsh.exe -NoLogo -ExecutionPolicy Bypass -NoExit -Command \". '\\\\?\\C:\\Users\\hnaganur\\AppData\\Local\\Programs\\GitHub Copilot\\terminal-integration\\shellIntegration.ps1'\"\0"` in cwd `Some("C:\\Users\\hnaganur\\.copilot\\chats\\fd4d9613-e8fd-4251-b269-5486833858ba\0")` failed: Access is denied. (os error 5)
```

### Steps to reproduce

- Launch GitHub Copilot app
- Open or navigate to a chat session
- Attempt to open a terminal

### Expected behavior

Terminal should initialize successfully and display a ready PowerShell prompt.

### Additional context

- **Error Code:** OS error 5 (Access is denied)
- **Shell:** PowerShell (pwsh.exe)
- **Environment:** Windows
- **Attempted Integration:** Shell integration script at `terminal-integration\shellIntegration.ps1`
- **Working Directory:** `.copilot\chats\fd4d9613-e8fd-4251-b269-5486833858ba`

从 GitHub Copilot 应用中使用 pwsh.exe 和报告的工作目录 .copilot\chats，在 Windows 上复现该故障。先从终端启动路径和 terminal-integration\shellIntegration.ps1 开始，然后验证打开终端时不会出现 OS 错误 5，并显示可用的 PowerShell 提示符。

由索引模型根据 Issue 内容生成。