Generated partial App class in XamlTypeInfo.g.cs has fixed "public" access modifier

[aries-zhang]

Describe the bug

When a WinUI 3 app has the CA1515: Consider making public types internal code rule enabled, the App class can not be made internal due to its partial class generated in XamlTypeInfo.g.cs is made public.

Steps to reproduce the bug

1. Create a blank WinUI 3 app with VisualStudio's WinUI 3 app template.
2. Go to App.xaml.cs, change the App class's access modifier from public to internal.
3. Build the project.
4. Build fails with the following error:

Error (active)	CS0262	Partial declarations of 'App' have conflicting accessibility modifiers

Expected behavior

App should be able to successfully build when change App to internal.

Screenshots

No response

NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

Windows version

Windows 11 (24H2): Build 26100

Additional context

Upon search there are 3 generated partial classes for App. While the following two do not have access modifier:
partial class App : global::Microsoft.UI.Xaml.Application

.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\Views\App.g.cs
.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\Views\App.g.i.cs

the generated partial class in
.\obj\x64\Debug\net8.0-windows10.0.22621.0\win-x64\XamlTypeInfo.g.cs
has public partial class App : global::Microsoft.UI.Xaml.Markup.IXamlMetadataProvider.