Avoid throwing exceptions after resource disposal

[runceel]

Changed the PropertyChanged method in the PropertyChangedObservable class to return instead of throwing an InvalidOperationException when _isDisposed is true. Added a return check in the SourcePropertyChangedEventHandler method of the PropertyPathNode class when _isDisposed is true. Changed the PropertyChanged method in the SimplePropertyObservable class to return instead of throwing an InvalidOperationException when _isDisposed is true.

[Copilot]

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.