ExceptionHandling.RaiseAppDomainUnhandledExceptionEvent: Prevent handlers from being invoked multiple times

This issue is tracking follow-up work from PR conversation here: https://github.com/dotnet/runtime/pull/117832#discussion_r2216958825

We want to prevent multiple AppContext.OnhandledException callbacks triggered via multi-threaded calls to ExceptionHandling.RaiseAppDomainUnhandledExceptionEvent