Initial ConfigureInstallType implementation

Copilot prompt:

Implement the #method:'Microsoft.DotNet.Tools.Bootstrapper.DotnetInstaller.ConfigureInstallType':4479-4606  method.  If the install type is user, remove any other folder with dotnet in it from the PATH and add the dotnetRoot to the PATH.  Also set the DOTNET_ROOT environment variable to dotnetRoot.

If the install type is Admin, unset DOTNET_ROOT, and add dotnetRoot to the path (removing any other dotnet folder from the path).

If the install type is None, unset DOTNET_ROOT and remove any dotnet folder from the path.

For any other install type, throw an ArgumentException.

Author: dsplaisted

src/Installer/dnup/DotnetInstaller.cs

@@ -103,5 +103,42 @@ public GlobalJsonInfo GetGlobalJsonInfo(string initialDirectory)
 
     public void InstallSdks(string dotnetRoot, ProgressContext progressContext, IEnumerable<string> sdkVersions) => throw new NotImplementedException();
     public void UpdateGlobalJson(string globalJsonPath, string? sdkVersion = null, bool? allowPrerelease = null, string? rollForward = null) => throw new NotImplementedException();
-    public void ConfigureInstallType(SdkInstallType installType, string? dotnetRoot = null) => throw new NotImplementedException();
+
+    public void ConfigureInstallType(SdkInstallType installType, string? dotnetRoot = null)
+    {
+        // Get current PATH
+        var path = Environment.GetEnvironmentVariable("PATH", EnvironmentVariableTarget.User) ?? string.Empty;
+        var pathEntries = path.Split(Path.PathSeparator, StringSplitOptions.RemoveEmptyEntries).ToList();
+        // Remove all entries containing "dotnet" (case-insensitive)
+        pathEntries = pathEntries.Where(p => !p.Contains("dotnet", StringComparison.OrdinalIgnoreCase)).ToList();
+
+        switch (installType)
+        {
+            case SdkInstallType.User:
+                if (string.IsNullOrEmpty(dotnetRoot))
+                    throw new ArgumentNullException(nameof(dotnetRoot));
+                // Add dotnetRoot to PATH
+                pathEntries.Insert(0, dotnetRoot);
+                // Set DOTNET_ROOT
+                Environment.SetEnvironmentVariable("DOTNET_ROOT", dotnetRoot, EnvironmentVariableTarget.User);
+                break;
+            case SdkInstallType.Admin:
+                if (string.IsNullOrEmpty(dotnetRoot))
+                    throw new ArgumentNullException(nameof(dotnetRoot));
+                // Add dotnetRoot to PATH
+                pathEntries.Insert(0, dotnetRoot);
+                // Unset DOTNET_ROOT
+                Environment.SetEnvironmentVariable("DOTNET_ROOT", null, EnvironmentVariableTarget.User);
+                break;
+            case SdkInstallType.None:
+                // Unset DOTNET_ROOT
+                Environment.SetEnvironmentVariable("DOTNET_ROOT", null, EnvironmentVariableTarget.User);
+                break;
+            default:
+                throw new ArgumentException($"Unknown install type: {installType}", nameof(installType));
+        }
+        // Update PATH
+        var newPath = string.Join(Path.PathSeparator, pathEntries);
+        Environment.SetEnvironmentVariable("PATH", newPath, EnvironmentVariableTarget.User);
+    }
 }