Fix Narrator silence on PipsPager page navigation (#2093)

## Fix Narrator announcement for PipsPager page changes

- [x] Initialize TestPipsPager2 with NumberOfPages property
- [x] Implement SelectedIndexChanged event handler for narrator
announcements
- [x] Use UIHelper.AnnounceActionForAccessibility for programmatic
narrator announcements
- [x] Enhance announcement format to include total page count
- [x] Extract notification ID to a constant for maintainability
- [x] Run code review to validate changes
- [x] Run CodeQL security check

## Summary

This PR fixes an accessibility issue where the Narrator screen reader
remained silent when users clicked the Next/Previous page buttons on the
PipsPager control in the WinUI 3 Gallery app.

### Changes Made:

1. **Initialized PipsPager**: Set NumberOfPages="10" on TestPipsPager2
to provide a working example
2. **Implemented event handler**: Added SelectedIndexChanged handler
that uses `UIHelper.AnnounceActionForAccessibility()` to
programmatically announce page changes to screen readers
3. **Enhanced announcement format**: Changed from "Page X selected" to
"Page X of Y selected" to provide users with context about their
position and the total number of pages
4. **Code quality improvement**: Extracted notification ID to a private
constant (`PipsPagerPageChangeNotificationId`) for better
maintainability and to prevent typos
5. **No visible UI changes**: Uses AutomationPeer.RaiseNotificationEvent
internally (via UIHelper) to announce without adding visible text
elements

### Technical Implementation:

The solution uses `UIHelper.AnnounceActionForAccessibility()` which
internally calls `AutomationPeer.RaiseNotificationEvent()` with
`AutomationNotificationKind.ActionCompleted`. This approach follows the
pattern used throughout the WinUI Gallery codebase (AppBarButtonPage,
BreadcrumbBarPage, ClipboardPage, etc.) for making narrator
announcements without requiring visible UI elements.

### Accessibility Impact:

Users who rely on screen readers will now hear "Page X of Y selected"
when navigating through pages using the Next/Previous buttons, meeting
MAS 4.2.12 accessibility requirements, without any visible UI changes to
the PipsPager control. The enhanced format provides better context by
including the total page count.

<!-- START COPILOT ORIGINAL PROMPT -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Upon selecting Next page/Previous page, Narrator remains
silent</issue_title>
> <issue_description>### Which version of the app?
> 
> WinUI 3 Gallery
> 
> ### Description
> 
> Environment Details:
> OS Version: 24H2 (OS Build 26100.4315)
> App: WinUI 3 Gallery
> App Version: 2.6.0.0 (SDK 1.7)
> AT: Color Contrast
> 
> Repro Steps:
> Launch the WinUI 3 Gallery app.
> Navigate to search box using tab key.
> Type 'Pipspager' in search box and activate it. 
> Now navigate to 'Next/Previous page' button under 'Pipspager with
options to change its orientation and button visibility'. And activate
it. Observe Narrator announcement.
> Actual Result:
> Upon selecting Next page/Previous page, Narrator remains silent
Instead of announcing the appeared page information and focused element
information.
> 
> Expected Result:
> Upon selecting Next page/Previous page, Narrator should not remain
silent, Instead it should announce the selected page Like, Page 5
selected.
> 
> User impact:
> Users who rely on Screen reader will face difficulty in understanding
the updated page information.
> 
> MAS Reference Link: [MAS 4.2.12 – Change
Notification](https://aka.ms/MAS4.2.12)
> 
> Trap ID: [1.9 - Feedback Failure](https://aka.ms/FeedbackFailure)
> 
> “Have feedback on bugs logged, please tag bug as “A11yRCA” and add
your feedback in the comment section.”
> 
> ### Screenshots
> 
> _No response_
> 
> ### Windows version
> 
> Windows 11 24H2 (26100)
> 
> ### Additional context
> 
> _No response_</issue_description>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> </comments>
> 


</details>



<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #2086

<!-- START COPILOT CODING AGENT TIPS -->
---

💬 We'd love your input! Share your thoughts on Copilot coding agent in
our [2 minute survey](https://gh.io/copilot-coding-agent-survey).

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ayushjai19 <244442986+ayushjai19@users.noreply.github.com>

Author: Copilot

WinUIGallery/Samples/ControlPages/PipsPagerPage.xaml

@@ -51,7 +51,10 @@
         </controls:ControlExample>
         <controls:ControlExample x:Name="Example2" HeaderText="PipsPager with options to change its orientation and button visibility.">
             <controls:ControlExample.Example>
-                <PipsPager x:Name="TestPipsPager2" />
+                <PipsPager
+                    x:Name="TestPipsPager2"
+                    NumberOfPages="10"
+                    SelectedIndexChanged="TestPipsPager2_SelectedIndexChanged" />
             </controls:ControlExample.Example>
             <controls:ControlExample.Options>
                 <StackPanel>

WinUIGallery/Samples/ControlPages/PipsPagerPage.xaml.cs

@@ -3,11 +3,14 @@
 
 using Microsoft.UI.Xaml.Controls;
 using System.Collections.Generic;
+using WinUIGallery.Helpers;
 
 namespace WinUIGallery.ControlPages;
 
 public sealed partial class PipsPagerPage : Page
 {
+    private const string PipsPagerPageChangeNotificationId = "PipsPagerPageChangeNotificationId";
+
     public List<string> Pictures = new List<string>()
     {
         "ms-appx:///Assets/SampleMedia/LandscapeImage1.jpg",
@@ -24,6 +27,14 @@ public PipsPagerPage()
         this.InitializeComponent();
     }
 
+    private void TestPipsPager2_SelectedIndexChanged(PipsPager sender, PipsPagerSelectedIndexChangedEventArgs args)
+    {
+        int pageNumber = sender.SelectedPageIndex + 1; // Convert 0-based index to 1-based page number
+        int totalPages = sender.NumberOfPages;
+        string announcement = $"Page {pageNumber} of {totalPages} selected";
+        UIHelper.AnnounceActionForAccessibility(sender, announcement, PipsPagerPageChangeNotificationId);
+    }
+
     private void OrientationComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
     {
         string? orientation = e.AddedItems[0].ToString();