Use COMSPEC environment variable for Windows shell path

Co-authored-by: Malcolmnixon <1863707+Malcolmnixon@users.noreply.github.com>

Author: Copilot

test/DemaConsulting.DotnetToolWrapper.Tests/IntegrationTests.cs

@@ -93,7 +93,14 @@ private static void CreateConfigFile(string program, string? directory = null)
     /// <returns>Shell program name</returns>
     private static string GetShellProgram()
     {
-        return RuntimeInformation.IsOSPlatform(OSPlatform.Windows) ? "cmd.exe" : "/bin/sh";
+        if (RuntimeInformation.IsOSPlatform(OSPlatform.Windows))
+        {
+            // Use COMSPEC environment variable to get full path to cmd.exe
+            var comspec = Environment.GetEnvironmentVariable("COMSPEC");
+            return comspec ?? throw new InvalidOperationException("COMSPEC environment variable not found");
+        }
+        
+        return "/bin/sh";
     }
 
     /// <summary>