ApplicationIcon for WinUI3 Unpackaged version

Nobody has claimed this yet.

### Describe the bug

Hello,

Why doesn't the parameter `ApplicationIcon` in the .csproj file work as its name suggests with a WinUI3 Unpackaged App ?

MyProj.csproj
```csproj
Assets\icon_name.ico
```

I have to use a small snippet of code to fix this "bug" :
```csharp
private void LoadIcon(string iconName)
{
this.AppWindow.SetIcon(Path.Combine(AppContext.BaseDirectory, $"Assets/{iconName}.ico"));
}
```

When the LoadIcon function is not called, my application takes the default icon (Win32)

Here’s how it appears on the taskbar:
> 

And here’s how it looks on the window bar:
> 

Am I doing something wrong or is it a bug ?

Thanks
Quentin

### Steps to reproduce the bug

1/ Set the ApplicationIcon parameter on the XAML file
```csproj
Assets\icon_name.ico
```

2/ Run/Launch the application

Here’s how it appears on the taskbar:
> 

And here’s how it looks on the window bar:
> 

### Expected behavior

_No response_

### Screenshots


### NuGet package version

Windows App SDK 1.6.3: 1.6.250108002

### Packaging type

Unpackaged

### Windows version

WIN11 10.0.26100.0

### IDE

Visual Studio 2022

### Additional context

_No response_

Start by reproducing the WinUI 3 unpackaged app with ApplicationIcon set in the .csproj, then compare its behavior with the LoadIcon snippet using AppWindow.SetIcon. Trace how the project setting is handled at launch; done means the configured icon appears on both the taskbar and window bar without the workaround.

Written by the indexing model from the issue text.