Add a read only subscript to ObservableModel (#358)

https://square.slack.com/archives/CBZJ5V163/p1751313246347059
UI-8947

When you have a screen with a model type of
`ActionModel<MyWorkflow.State, MyWorkflow.Action>`, you were unable to
use a `let` or computed `var` property on your state model, and
reference it on that screen. If you did you would get a compiler error
of `Cannot assign to property: 'enableDoneButton' is a 'let' constant`,
even if you were only trying to read the property.


## Checklist

- [ ] Unit Tests - There aren't any tests of `ObservableModel` in the
codebase currently. As I'm not very familiar with this codebase, I don't
know how to really make a useful test here.
- [ ] UI Tests
- [ ] Snapshot Tests (iOS only)
- [ ] I have made corresponding changes to the documentation

Author: amayers

WorkflowSwiftUI/Sources/ObservableModel.swift

@@ -147,4 +147,9 @@ extension ObservableModel {
             accessor.sendValue { $0[keyPath: keyPath] = newValue }
         }
     }
+
+    /// Allows dynamic member lookup to read state through the accessor.
+    public subscript<T>(dynamicMember keyPath: KeyPath<State, T>) -> T {
+        accessor.state[keyPath: keyPath]
+    }
 }