[.NET] CreateProcess fails when Copilot CLI is installed via npm (copilot.cmd)

## Problem

On Windows, when the Copilot CLI is installed via npm, the only executables on PATH are `copilot` (extensionless) and `copilot.cmd`. There is no `copilot.exe`.

The .NET SDK's `StartCliServerAsync` uses `ProcessStartInfo` with `UseShellExecute = false`, which calls `CreateProcess` under the hood. `CreateProcess` cannot resolve `.cmd`/`.bat` files — it only finds native executables (`.exe`, `.com`). This results in:

```
System.ComponentModel.Win32Exception (2): An error occurred trying to start process 'copilot'
with working directory '...'. The system cannot find the file specified.
at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)
at GitHub.Copilot.SDK.CopilotClient.StartCliServerAsync(...)
```

Running `copilot` in a terminal works fine because PowerShell/cmd.exe use PATHEXT to resolve `.cmd` wrappers. But the SDK's `CreateProcess` does not.

**Output of `where.exe copilot` on affected machines:**
```
C:\ProgramData\global-npm\copilot
C:\ProgramData\global-npm\copilot.cmd
```

## This was already fixed in the Python SDK

PR #338 fixed the same issue for the Python SDK by using `shutil.which()` to resolve the full CLI path before spawning the subprocess.

## Suggested fix for .NET

The SDK already has `ResolveCliCommand` which handles `.js` files by prepending `node`:

```csharp
private static (string FileName, IEnumerable Args) ResolveCliCommand(string cliPath, IEnumerable args)
{
if (cliPath.EndsWith(".js", StringComparison.OrdinalIgnoreCase))
{
return ("node", new[] { cliPath }.Concat(args));
}
return (cliPath, args);
}
```

A similar approach for `.cmd` files (prepending `cmd.exe /c`) would work. Alternatively, resolve the PATH using PATHEXT-aware logic before passing to `ProcessStartInfo`.

## Environment

- **SDK**: GitHub.Copilot.SDK 0.1.32 (NuGet)
- **OS**: Windows 11
- **CLI install method**: npm (global)