Can't specify `x:DataType` of DataTemplate in ResourceDictionary

### Describe the bug

I have a templated control that has a property of type `Windows.Foundation.Collections.IObservableVector Items{ get; };` that's bound to a `ListView.ItemsSource` and I can't specify the `x:DataType` in the `DataTemplate`. It gives
```
Themes\Generic.xaml(12,43): XamlCompiler error WMC0612: The XAML Binary Format (XBF) generator reported syntax error '0x09C4' : Property Not Found
```

### Steps to reproduce the bug

1. Create a new C++WinUI3 project
2. Create a new templated control with this idl, write a dummy code-behind
```idl
[bindable]
[default_interface]
runtimeclass MyControl : Microsoft.UI.Xaml.Controls.Control
{
MyControl();
Windows.Foundation.Collections.IObservableVector Items{ get; };
}
```
3. In `Themes/Generc.xaml`, add this style
```xml

<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:MyControl">
<ListView ItemsSource="{Binding Items, RelativeSource={RelativeSource Mode=TemplatedParent}}">
<ListView.ItemTemplate>
<DataTemplate x:DataType="x:Int32"/>
</ListView.ItemTemplate>
</ListView>
</ControlTemplate>
</Setter.Value>
</Setter>

```

### Expected behavior

Builds fine

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.7.1: 1.7.250401001

### Windows version

Windows 11 (24H2): Build 26100

### Additional context
[Repro](https://github.com/HO-COOH/WinUI-Bug/tree/main/28-CannotUseInt32AsDataType)

Start with the Themes/Generic.xaml example in the issue and the linked 28-CannotUseInt32AsDataType repro project. Run the C++/WinUI 3 project build with the DataTemplate x:DataType="x:Int32" declaration, then trace the XAML compiler handling of this ResourceDictionary case. Done means the repro builds without WMC0612 and the expected template remains usable.

Written by the indexing model from the issue text.