AppInstance redirection has potential to hang forever

Due to lack of full support for CommandLineActivatedEventArgs - https://github.com/microsoft/WindowsAppSDK/issues/6431 - I've been augmenting app instance redirection with my own version for that case. One thing I've noticed in examining the code is that it's possible for an instance to hang indefinitely if the instance it's redirecting to is in the process of shutting down. It would be quite easy to mitigate against this.

In this file https://github.com/microsoft/WindowsAppSDK/blob/main/dev/AppLifecycle/AppInstance.cpp, all you would need to do is signal the `cleanupEvent` (if it exists) in the `onInstanceTerminated` handler, and also check that the process is still open just after creating the `cleanupEvent` (else fail). In an ideal world you might also communicate the error to the waiting process (I guess you might need a second event to signal the error or use a memory mapped file), but in my opinion it's probably better for the waiting process to carry on as normal (usually just exit) rather than hang indefinitely in this case.

Start in dev/AppLifecycle/AppInstance.cpp and trace the app-instance redirection path through cleanupEvent and the onInstanceTerminated handler. Validate the shutdown race described in the issue; the work is done when a target instance closing during redirection cannot leave the waiting instance blocked indefinitely.

Written by the indexing model from the issue text.