Fix: UIA tree not visible for Avalonia content embedded via WinFormsAvaloniaControlHost (#21422)

* [Automation] Add EmbeddableControlRootAutomationPeer tests

* [Automation] Implement IRootProvider on EmbeddableControlRootAutomationPeer

Without IRootProvider, AutomationNode.Create falls back to the plain
AutomationNode which doesn't implement IRawElementProviderFragmentRoot
and returns null from GetHostRawElementProvider. As a result,
UiaReturnRawElementProvider responds with E_FAIL for the WM_GETOBJECT
sent to the embedded HWND, and the Avalonia automation tree is invisible
to UIA clients (Inspect.exe, Narrator, FlaUI etc.) when the control is
hosted via WinFormsAvaloniaControlHost.

src/Avalonia.Controls/Automation/Peers/EmbeddableControlRootAutomationPeer.cs

@@ -5,11 +5,12 @@ using Avalonia.Automation.Peers;
 using Avalonia.Automation.Provider;
 using Avalonia.Controls.Embedding;
 using Avalonia.Input;
+using Avalonia.Platform;
 using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Automation.Peers
 {
-    public class EmbeddableControlRootAutomationPeer : ContentControlAutomationPeer, IEmbeddedRootProvider
+    public class EmbeddableControlRootAutomationPeer : ContentControlAutomationPeer, IEmbeddedRootProvider, IRootProvider
     {
         private Control? _focus;
 
@@ -24,6 +25,8 @@ namespace Avalonia.Controls.Automation.Peers
 
         public new EmbeddableControlRoot Owner => (EmbeddableControlRoot)base.Owner;
 
+        ITopLevelImpl? IRootProvider.PlatformImpl => Owner.PlatformImpl;
+
         public event EventHandler? FocusChanged;
 
         public AutomationPeer? GetFocus() => _focus is object ? GetOrCreate(_focus) : null;

tests/Avalonia.Controls.UnitTests/Automation/EmbeddableControlRootAutomationPeerTests.cs

@@ -0,0 +1,47 @@
+using Avalonia.Automation.Peers;
+using Avalonia.Automation.Provider;
+using Avalonia.UnitTests;
+using Xunit;
+
+#nullable enable
+
+namespace Avalonia.Controls.UnitTests.Automation
+{
+    public class EmbeddableControlRootAutomationPeerTests : ScopedTestBase
+    {
+        [Fact]
+        public void Peer_Provides_IRootProvider()
+        {
+            using var services = new CompositorTestServices();
+            var peer = ControlAutomationPeer.CreatePeerForElement(services.TopLevel);
+
+            var rootProvider = peer.GetProvider<IRootProvider>();
+
+            Assert.NotNull(rootProvider);
+            Assert.Same(peer, rootProvider);
+        }
+
+        [Fact]
+        public void Peer_Still_Provides_IEmbeddedRootProvider()
+        {
+            using var services = new CompositorTestServices();
+            var peer = ControlAutomationPeer.CreatePeerForElement(services.TopLevel);
+
+            var embeddedRootProvider = peer.GetProvider<IEmbeddedRootProvider>();
+
+            Assert.NotNull(embeddedRootProvider);
+        }
+
+        [Fact]
+        public void IRootProvider_PlatformImpl_Returns_Owner_PlatformImpl()
+        {
+            using var services = new CompositorTestServices();
+            var peer = ControlAutomationPeer.CreatePeerForElement(services.TopLevel);
+
+            var rootProvider = peer.GetProvider<IRootProvider>();
+
+            Assert.NotNull(rootProvider);
+            Assert.Same(services.TopLevel.PlatformImpl, rootProvider!.PlatformImpl);
+        }
+    }
+}