Called when the user indicates that they are done editing the text in the field.
By default, onSubmitted is called after onChanged when the user has finalized editing; or, if the default behavior has been overridden, after onEditingComplete. See onEditingComplete for details.
The following is the recommended way to trigger onSubmitted in a test:
tester.sendKeyEvent will not
trigger onSubmitted. This is because on a real device, the engine
translates the enter key to a done action, but
the key to the framework only.
final ValueChanged<String>? onSubmitted;