Powershell tool requires `pwsh` and fails if I do not have it installed

### Describe the bug

```
{"type":"tool.execution_complete","data":{"toolCallId":"toolu_vrtx_015T42XEjysw1jwbYmh9uEs2","success":false,"error":{"message":"","code":"failure"},"toolTelemetry":{"properties":{"customTimeout":"true","executionMode":"sync"},"metrics":{"commandTimeout":180000}}},"id":"a5b01fca-e4e0-4d33-8ab7-6fde82c12015","timestamp":"2025-12-27T02:35:44.276Z","parentId":"6ef8d167-5161-4415-9eb7-76a21fe2f1a1"}
```

Repro steps:
```
Welcome to GitHub Copilot CLI
Version 0.0.372 · Commit 5534560

Copilot can write, test and debug code right from your terminal. Describe a task to get started or enter ? for help. Copilot uses AI, check for mistakes.

● Logged in as user: Tonychen0227

● Connected to GitHub MCP Server

> Print hello world for me in powershell
```

Fails if `pwsh` is not available on the machine.

### Affected version

0.0.372

### Steps to reproduce the behavior

1. Type 'Print hello world for me in powershell' into Copilot on a machine without PWSH installed
2. Fails

### Expected behavior

Should fallback to `powershell` instead of `pwsh`

### Additional context

_No response_