win32: Don't raise TextInput event when KeyDown was handled.

On win32, returning handled from `WM_KEYDOWN` doesn't automatically prevent a `WM_CHAR` message, resulting in #5849.

Fix this by setting a flag after `WM_KEYDOWN` is handled which will make the following `WM_CHAR` message be ignored.

This is the second attempt at fixing this after it was decided that https://github.com/AvaloniaUI/Avalonia/pull/7351 was not the right approach.

Fixes #5849

src/Windows/Avalonia.Win32/WindowImpl.AppWndProc.cs

@@ -151,8 +151,8 @@ namespace Avalonia.Win32
                     }
                 case WindowsMessage.WM_CHAR:
                     {
-                        // Ignore control chars
-                        if (ToInt32(wParam) >= 32)
+                        // Ignore control chars and chars that were handled in WM_KEYDOWN.
+                        if (ToInt32(wParam) >= 32 && !_ignoreWmChar)
                         {
                             e = new RawTextInputEventArgs(WindowsKeyboardDevice.Instance, timestamp, _owner,
                                 new string((char)ToInt32(wParam), 1));
@@ -519,6 +519,15 @@ namespace Avalonia.Win32
             {
                 Input(e);
 
+                if ((WindowsMessage)msg == WindowsMessage.WM_KEYDOWN)
+                {
+                    // Handling a WM_KEYDOWN message should cause the subsequent WM_CHAR message to
+                    // be ignored. This should be safe to do as WM_CHAR should only be produced in
+                    // response to the call to TranslateMessage/DispatchMessage after a WM_KEYDOWN
+                    // is handled.
+                    _ignoreWmChar = e.Handled;
+                }
+
                 if (e.Handled)
                 {
                     return IntPtr.Zero;

src/Windows/Avalonia.Win32/WindowImpl.cs

@@ -90,6 +90,7 @@ namespace Avalonia.Win32
         private bool _shown;
         private bool _hiddenWindowIsParent;
         private uint _langid;
+        private bool _ignoreWmChar;
 
         public WindowImpl()
         {