Announce Theme Transitions page action button updates (#2146)

<!--- Provide a general summary of your changes in the Title above -->

## Description
Added accessibility announcements to the ThemeTransitionPage using
UIHelper.AnnounceActionForAccessibility so that screen readers properly
announce the result of user-initiated actions (e.g., adding/deleting
items, refreshing content, repositioning elements, adding/clearing
rectangles).

Also changed var to explicit int type for value in
EntranceAddButton_Click to comply with project coding conventions.

## Motivation and Context
Screen readers did not announce the outcome of button actions on the
Theme Transition sample page, making it inaccessible for users relying
on assistive technology.

Bug:
[https://microsoft.visualstudio.com/OS/_workitems/edit/60914518](vscode-file://vscode-app/c:/Program%20Files/Microsoft%20VS%20Code/ce099c1ed2/resources/app/out/vs/code/electron-browser/workbench/workbench.html)

## How Has This Been Tested?
Manually verified by deploying WinUI Gallery Dev app locally that each
button action on the Theme Transition page triggers a screen reader
announcement with the expected text.
Confirmed existing functionality (add, delete, reposition, entrance
transitions) is unchanged.

## Screenshots (if appropriate):


https://github.com/user-attachments/assets/4d395977-5d9d-4233-9689-269d5c65802b

## Types of changes
<!--- What types of changes does your code introduce? Put an `x` in all
the boxes that apply: -->
- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)

---------

Co-authored-by: Shubhika Shubhavee <sshubhavee@microsoft.com>

Author: sshubhavee

WinUIGallery/Samples/ControlPages/ThemeTransitionPage.xaml.cs

@@ -7,6 +7,7 @@
 using Microsoft.UI.Xaml.Shapes;
 using System;
 using System.Collections.Generic;
+using WinUIGallery.Helpers;
 
 namespace WinUIGallery.ControlPages;
 
@@ -41,6 +42,7 @@ private void ClosePopupButton_Click(object sender, RoutedEventArgs e)
     private void ContentRefreshButton_Click(object sender, RoutedEventArgs e)
     {
         AddItemsToContentListView(true);
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, "Data refreshed.", "ContentRefreshNotificationId");
     }
 
     private void AddItemsToContentListView(bool ShowDifferentContent = false)
@@ -58,38 +60,49 @@ private void AddButton_Click(object sender, RoutedEventArgs e)
     {
         AddRemoveListView.Items.Add(new ListViewItem() { Content = "New Item " + _itemCount.ToString() });
         _itemCount++;
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, "Item added.", "AddDeleteItemAddedNotificationId");
     }
 
     private void DeleteButton_Click(object sender, RoutedEventArgs e)
     {
         if (AddRemoveListView.Items.Count > 0)
+        {
             AddRemoveListView.Items.RemoveAt(0);
+            UIHelper.AnnounceActionForAccessibility((UIElement)sender, "Item deleted.", "AddDeleteItemDeletedNotificationId");
+        }
     }
 
     private void RepositionButton_Click(object sender, RoutedEventArgs e)
     {
         MiddleElement.Visibility = MiddleElement.Visibility == Visibility.Visible ? Visibility.Collapsed : Visibility.Visible;
+        string announcement = MiddleElement.Visibility == Visibility.Visible ? "Element restored." : "Element repositioned.";
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, announcement, "RepositionNotificationId");
     }
 
     private void EntranceAddButton_Click(object sender, RoutedEventArgs e)
     {
-        var value = Convert.ToInt32((sender as Button)?.Tag);
+        int value = Convert.ToInt32((sender as Button)?.Tag);
 
         for (int i = 0; i < value; i++)
         {
             Thickness thickness = new Thickness(5.0);
             EntranceStackPanel.Children.Add(new Rectangle() { Width = 50, Height = 50, Margin = thickness, Fill = new SolidColorBrush(Microsoft.UI.Colors.LightBlue) });
         }
+
+        string announcement = value == 1 ? "Added 1 rectangle." : $"Added {value} rectangles.";
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, announcement, "EntranceAddNotificationId");
     }
 
     private void EntranceClearButton_Click(object sender, RoutedEventArgs e)
     {
         EntranceStackPanel.Children.Clear();
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, "All rectangles cleared.", "EntranceClearNotificationId");
     }
 
     private void AddDeleteButton_Click(object sender, RoutedEventArgs e)
     {
         AddButton_Click(sender, e);
         DeleteButton_Click(sender, e);
+        UIHelper.AnnounceActionForAccessibility((UIElement)sender, "Item added and item deleted.", "AddDeleteBothNotificationId");
     }
 }