maxAllowedSelections read as double in multiple_choice widget

[mwk24]

Describe the bug
MultipleChoiceState.maxAllowedSelections casts _json['maxAllowedSelections'] directly to int?, but the JSON map stores numeric values as doubles. When the field is present you get type 'double' is not a subtype of type 'int?' in type cast. Code reference: packages/genui/lib/src/catalog/core_widgets/multiple_choice.dart:44-45.

To Reproduce

1. Configure a multiple choice widget that sets maxAllowedSelections (e.g. 3) and let it round-trip through the GenUI JSON map.
2. The JSON is decoded so numeric literals are double (e.g. 3.0).
3. When the getter runs, the cast to int? throws and the widget fails to render.

Expected behavior
maxAllowedSelections should accept numeric JSON input and convert it to an int. Reading it as num? and calling .toInt() would avoid the crash.

Additional context
The same pattern might exist in other widget configs that assume JSON numbers deserialize as int. Fixing this getter would unblock multiple choice responses that limit the selection count.

[jacobsimionato]

Thank you for this fantastic bug description! Marking triaged and P2.