App activation fires multiple times when launching multiple files

### Describe the bug

I have an app that can open image files. Users can associate image file endings (e.g. `.jpg`) with my app or open the image files in my app using the context menu in the Windows explorer and use "open with...".
When users select multiple files (e.g. 5 image files) and then open them all in my app with a single click on the "Open" option in the context menu, my app is activated 5 times and in with each activation, the `FileActivatedEventArgs.Files` contains all 5 files in each activation.

### Steps to reproduce the bug

```

.jpg
.jpeg
...

Images

```

[STAThread]
static void Main()
{
var keyInstance = AppInstance.FindOrRegisterForKey("main");
if (keyInstance.IsCurrent)
{
keyInstance.Activated += (_, e) =>
{
// handle activation here
};
Application.Start(p =>
{
var context = new DispatcherQueueSynchronizationContext(DispatcherQueue.GetForCurrentThread());
SynchronizationContext.SetSynchronizationContext(context);
_ = new App();
});
}
else { keyInstance.RedirectActivationToAsync(AppInstance.GetCurrent().GetActivatedEventArgs()).GetAwaiter().GetResult(); }
}

### Expected behavior

My app should only be activated once.

### Screenshots

_No response_

### NuGet package version

Windows App SDK 1.6.3: 1.6.250108002

### Packaging type

Packaged (MSIX)

### Windows version

Windows 11 version 24H2 (22621, October 2024 Update)

### IDE

Visual Studio 2022

### Additional context

This problem was not present in UWP. My app was only activated once in UWP, even when multiple files were opened

Start at the AppInstance.FindOrRegisterForKey and Activated handling in the Main example, then reproduce the packaged Windows App SDK 1.6.3 case by opening multiple image files from Windows Explorer. Trace how FileActivatedEventArgs.Files is delivered across activations; done means one activation for the selection, with all selected files available once.

Written by the indexing model from the issue text.