x:Bind with cast fails to compile with C++

### Describe the bug

Got
```
error C2665: 'winrt::to_hstring': no overloaded function could convert all the argument types
```

The exact same code works in C# project

### Steps to reproduce the bug

1. Create a new winui3 C++ project, use this xaml
```xml


A
B



```
2. Build and observe the compile error

### Expected behavior

_No response_

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.7 Experimental 3: 1.7.250127003-experimental3

### Windows version

Windows 11 (24H2): Build 26100

### Additional context

_No response_

No repository file or test is named. Reproduce the XAML in a WinUI 3 C++ project, then trace the x:Bind cast compilation path and the reported winrt::to_hstring error; done means the sample builds and the cast binding displays the selected item as intended.

Written by the indexing model from the issue text.