If the current thread dispatcher is null return the app level dispatcher

[PureWeen]

Description of Change

Account for scenarios where the first call to the application level IDispatcher is from a background thread.

• We modify the logic of FindDispatcher to fall back to our ApplicationDispatcher container. This means if the user calls Application.Current.Dispatcher from a background thread they won't fail.
• We also added a coalesce inside the factory call for "IDispatcher".
  • The main purpose of this is to account for users calling IPlatformApplication.Current.Services.Get<IDispatcher>() from a background thread. Because we're no longer initializing that path via initializers we need to account for the case where a user is trying to resolve a scoped service from the static container.
  • If users try to request the window level dispatcher from a background thread, they will still get the correct dispatcher because now we're initializing that dispatcher whenever a scoped service is created. In the current stable version of NET8 (8.0.6) if a users first request for the dispatcher is from a background thread it'll return null, and they'll be stuck in a bad state.

[jsuarezruiz]

D:\a\_work\1\s\src\Compatibility\ControlGallery\src\Issues.Shared\TestPages\TestPages.cs(962,17): error CS0122: 'ApplicationDispatcher' is inaccessible due to its protection level [D:\a\_work\1\s\src\Compatibility\ControlGallery\test\WinUI.UITests\WinUI.UITests.csproj]

[PureWeen]

/rebase

[PureWeen]

iOS failures are unrelated