CmdPal: Fix RDP extension rejecting host:port connections (#45740)

## Summary

Fixes #45100

Uri.CheckHostName does not accept host:port strings (e.g.
localhost:3389), returning UriHostNameType.Unknown. This causes the RDP
extension to show an invalid hostname error when connecting to a local
forwarded port.

## Changes

- OpenRemoteDesktopCommand.cs - Strip port suffix before
Uri.CheckHostName validation. The full host:port is still passed to
mstsc /v:.
- FallbackRemoteDesktopItem.cs - Same port-aware validation so the
fallback correctly recognizes host:port queries and displays them in the
title.
- FallbackRemoteDesktopItemTests.cs - Added tests for localhost:3389 and
192.168.1.100:3390 inputs.

## Validation

Port detection uses LastIndexOf(':') + ushort.TryParse to safely
identify a trailing port number without affecting IPv6 addresses or
plain hostnames.

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/modules/cmdpal/Tests/Microsoft.CmdPal.Ext.RemoteDesktop.UnitTests/FallbackRemoteDesktopItemTests.cs

@@ -98,6 +98,48 @@ public class FallbackRemoteDesktopItemTests
         Assert.IsNull(command);
     }
 
+    [TestMethod]
+    public void UpdateQuery_WhenQueryIsHostnameWithPort_UsesFullHostPort()
+    {
+        // Arrange
+        var setup = CreateFallback();
+        var fallback = setup.Fallback;
+        const string hostPort = "localhost:3389";
+
+        // Act
+        fallback.UpdateQuery(hostPort);
+
+        // Assert
+        var expectedTitle = string.Format(CultureInfo.CurrentCulture, OpenHostCompositeFormat, hostPort);
+        Assert.AreEqual(expectedTitle, fallback.Title);
+        Assert.AreEqual(Resources.remotedesktop_title, fallback.Subtitle);
+
+        var command = fallback.Command as OpenRemoteDesktopCommand;
+        Assert.IsNotNull(command);
+        Assert.AreEqual(hostPort, GetCommandHost(command));
+    }
+
+    [TestMethod]
+    public void UpdateQuery_WhenQueryIsIPWithPort_UsesFullHostPort()
+    {
+        // Arrange
+        var setup = CreateFallback();
+        var fallback = setup.Fallback;
+        const string hostPort = "192.168.1.100:3390";
+
+        // Act
+        fallback.UpdateQuery(hostPort);
+
+        // Assert
+        var expectedTitle = string.Format(CultureInfo.CurrentCulture, OpenHostCompositeFormat, hostPort);
+        Assert.AreEqual(expectedTitle, fallback.Title);
+        Assert.AreEqual(Resources.remotedesktop_title, fallback.Subtitle);
+
+        var command = fallback.Command as OpenRemoteDesktopCommand;
+        Assert.IsNotNull(command);
+        Assert.AreEqual(hostPort, GetCommandHost(command));
+    }
+
     private static string GetCommandHost(OpenRemoteDesktopCommand command)
     {
         var field = typeof(OpenRemoteDesktopCommand).GetField("_rdpHost", BindingFlags.NonPublic | BindingFlags.Instance);