I don't have an example for you, but to answer your question more generally, I'd keep the text editing controllers on the UI side, that is, as variables in a stateful widget since those controllers belong to Flutter. The only piece of Flutter that I let into the state management layer is ValueNotifier.

However, any time that you have to process text (like validating the input on a form field), that might be something worth giving to the state management class to handle. That would allow you to create a simple unit test to test the validation logic rather than trying to set up some widget test.