VisualTreeHelper unable to traverse TeachingTip content

### Describe the bug

I am doing some customization using attached `DependencyProperty`, so I attached a `Loaded` event handler to a `TeachingTip` to traverse it with `VisualTreeHelper`, but it is unable to traverse the visual tree of it, giving no useful information.

### Why is this important?

This should work

### Steps to reproduce the bug
I have a `TeachingTip` defined in xaml
```xml
Click to open tip

```

0. Add a helper function to traverse a control and print it type name and name
```cpp
static void printVisualTree(winrt::Microsoft::UI::Xaml::FrameworkElement const& element)
{
if (!element)
return;

OutputDebugString(std::format(L"{}: {}\n", winrt::get_class_name(element), element.Name()).data());

auto const numChild = winrt::Microsoft::UI::Xaml::Media::VisualTreeHelper::GetChildrenCount(element);
for (auto i = 0; i < numChild; ++i)
{
printVisualTree(winrt::Microsoft::UI::Xaml::Media::VisualTreeHelper::GetChild(element, i).as());
}
}
```

1. Print out the visual tree in both `Loaded` and `LayoutUpdated` event

```cpp
void MainWindow::MyTip_Loaded(winrt::Windows::Foundation::IInspectable const& sender, winrt::Microsoft::UI::Xaml::RoutedEventArgs const& e)
{
auto teachingTip = sender.as();
OutputDebugString(L"==============Loaded==========\n");
printVisualTree(teachingTip);
}

void MainWindow::MyTip_LayoutUpdated(winrt::Windows::Foundation::IInspectable const& sender, winrt::Windows::Foundation::IInspectable const& e)
{
OutputDebugString(L"==============Layout Updated==========\n");
printVisualTree(MyTip());
}
```

### Actual behavior
Gives
```
==============Layout Updated==========
Microsoft.UI.Xaml.Controls.TeachingTip: MyTip
Microsoft.UI.Xaml.Controls.Border: Container
==============Loaded==========
Microsoft.UI.Xaml.Controls.TeachingTip: MyTip
Microsoft.UI.Xaml.Controls.Border: Container
```

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.8.2: 1.8.251003001

### Windows version

Windows 11 (24H2): Build 26100

### Additional context
[Full repro](https://github.com/HO-COOH/WinUI-Bug/tree/main/33-TeachingTipNullContent)

Run the linked full repro and inspect the TeachingTip behavior in the Loaded and LayoutUpdated handlers using VisualTreeHelper. Start with the TeachingTip, its Container Border, and the WinUI 3 Windows App SDK 1.8.2 context; done means the expected visual-tree traversal is established and the reported inability to reach the TeachingTip content is resolved or clearly documented.

Written by the indexing model from the issue text.