TreeView does not respect initial SelectedItem when initialized

### Describe the bug

I created a reproducible project [here](https://github.com/AndrewKeepCoding/TreeViewSelectedItemBugSample) but is something like the following:

```cs
public partial class ShellViewModel : ObservableObject
{
[ObservableProperty]
private string[] _items =
[
"Item 1",
"Item 2",
"Item 3",
];

[ObservableProperty]
private string? _selectedItem;

public ShellViewModel()
{
SelectedItem = Items[1];
}

[RelayCommand]
private void SelectFirstItem() => SelectedItem = Items.First();

[RelayCommand]
private void SelectLastItem() => SelectedItem = Items.Last();
}
```

```xaml

```

As you can see, the `TreeView` control is not respecting the bound `SelectedItem` right after it's initialized. It should have "Item 2" selected. And you can also see that the binding is not broken.

https://github.com/user-attachments/assets/352fae30-d9ff-4fb1-8b02-609922eaeccd

### Steps to reproduce the bug

Use the repo above.

### Expected behavior

`TreeView` respects `SelectedItem` when loaded.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

### Windows version

Windows Insider Build (xxxxx)

### Additional context

_No response_

Start with the linked TreeViewSelectedItemBugSample and reproduce the initialization behavior using the XAML SelectedItem binding shown in the issue. Compare TreeView initialization with the ListView, GridView, and NavigationView examples, then verify that Item 2 is selected when the control loads while later two-way binding still works.

Written by the indexing model from the issue text.