Prevent an additional world update after all windows have closed on Windows

[aloucks]

Objective

The fix in #18105 includes a check for running headless, but that allows for an extra world update during shutdown.

This commit checks if the AppExit event has been recorded and prevents the additional world update.

Before

2025-03-06T03:11:59.999679Z  INFO bevy_window::system: No windows are open, exiting    
2025-03-06T03:12:00.001942Z  INFO bevy_winit::system: Closing window 0v1
2025-03-06T03:12:00.012691Z  INFO bevy_window::system: No windows are open, exiting

After

2025-03-06T03:18:45.552243Z  INFO bevy_window::system: No windows are open, exiting    
2025-03-06T03:18:45.554119Z  INFO bevy_winit::system: Closing window 0v1

Testing

Ran window examples

• monitor_info continues to run after all windows are closed (it has ExitCondition::DontExit)
• window_settings invisible window creation works as expected
• multiple_windows exits after both windows are closed with a single exit message

[greeble-dev]

Tested on Win10/Nvidia. Tried the mentioned examples and a few others.

Side note: Is there a reason the monitor_info example is set to not close when all windows are closed? I assumed this was a bug at first. I tried removing the ExitCondition::DontExit and it worked fine, but I don't have multiple monitors.

[aloucks]

Side note: Is there a reason the monitor_info example is set to not close when all windows are closed? I assumed this was a bug at first. I tried removing the ExitCondition::DontExit and it worked fine, but I don't have multiple monitors.

That confused me on a previous PR. I also thought it was a bug at first. I think there needs to be an example that exercises that behavior, but I'm not sure that monitor_info is the right place. I didn't want to change the examples in this PR though.