accessibility: Ctrl+Home then Down arrow key go to system bar for WebView2 app

### Describe the bug

Seems WinUI2 is not well-integrated with WebView2 for accessibility issue in scan mode. Ctrl+Home will move focus to the top of the webpage content, but subsequent down arrow presses will navigate to the app system bar.

try [Win32 + WebView2](https://github.com/MicrosoftEdge/WebView2Samples/tree/main/GettingStartedGuides/Win32_GettingStarted), this issue don't repro. so this should be WinUI2 bug.

### Steps to reproduce the bug

1. Launch the sample app https://github.com/MicrosoftEdge/WebView2Samples/tree/main/GettingStartedGuides/WinUI2_GettingStarted/MyUWPGetStartApp

2. (optional) Update WinUI2 2.8.6, WebView2 1.0.2903.40

3. Turn On Narrator, "Tap" to one of page elements
4. "Caps Lock" + "Space" to turn on scan mode
5. Press Ctrl + Home to move to the top of the content.
6. Use the down arrow key to start navigating the content.
=> hear "MyUWPGetStartApp" from system bar
expect => the next element of the page content

### Expected behavior

**Actual**:
After pressing Ctrl+Home, the screen reader first moves focus to the top of the webpage content, but subsequent down arrow presses navigate to system bar like the title bar, minimize button, maximalize button

**Expected**:
Focus should remain within the content after using Ctrl+Home, and down arrow navigation should continue consistently from the top of the webpage content without jumping through other elements like the system title bar.

### Screenshots

_No response_

### NuGet package version

WinUI 2 - Microsoft.UI.Xaml 2.8.2

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

Reproduce the issue in the WinUI2_GettingStarted sample MyUWPGetStartApp with Narrator scan mode, then compare its behavior with the Win32 + WebView2 sample linked in the report. Done means Ctrl+Home followed by Down arrows keeps focus within the webpage content instead of moving through the system bar.

Written by the indexing model from the issue text.