Content of TabViewItem does not change after setting its Content property

### Describe the bug

```cpp
const TabViewItem item;
item.Header(box_value(L"New Tab"));
tabView().TabItems().Append(item);
tabView().SelectedItem(item);
const NewTab page;
page.Position(item, self.get());
item.Content(page);
```

In the code above, the Content property is set after adding `item` to `tabView().TabItems()`. However, this resulted in the content not being visible. Reordering the operations—setting `Content` before adding the item—resolved the issue.

Is this the expected behavior, or could it be a bug?

### Steps to reproduce the bug

Obvious

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

Windows App SDK 1.8 Experimental 4: 1.8.250702007-experimental4

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (26100, June 2025 Update)

### IDE

Visual Studio 2022

### Additional context

_No response_

No source file or test is named. Start by reproducing the issue with the provided C++ TabViewItem sequence, then inspect the TabViewItem Content behavior and existing tests; done means determining whether setting Content after insertion is supported and correcting the behavior if it is a bug.

Written by the indexing model from the issue text.