Stop passing State objects between widgets in LayoutExplorer

[srawlins]

Replaces passing FlexLayoutExplorerWidgetState down to VisualizeFlexChildren and FlexChildVisualizer with FlexLayoutExplorerScope, an InheritedWidget.

FlexLayoutExplorerScope provides descendants with scoped access to rootProperties, the entrance animation (entranceController and entranceCurve), and the necessary interaction callbacks (markAsDirty, onTap, and onDoubleTap). This eliminates passing mutable State objects down the widget tree while avoiding prop drilling and preserving all existing layout visualizer behaviors.

Fixes #2701

[gemini-code-assist]

Code Review

This pull request refactors the flex layout explorer by introducing FlexLayoutExplorerScope, an InheritedWidget used to share properties and callbacks down the widget tree. This successfully removes the need to pass the state object directly to child widgets. The review feedback recommends adding defensive null checks for the nullable objectGroup in FlexChildVisualizer to avoid potential runtime crashes from using the null-assertion operator.

[gemini-code-assist]

[MUST-FIX] Defensive Null Check for objectGroup

The objectGroup getter returns a nullable ObjectGroup?. Using the null-assertion operator (!) on it can lead to runtime crashes if the object group is null (e.g., during transitions or if the inspector service is disconnected).

We should safely bind objectGroup to a local variable, perform a null check, and return early if it is null.

  void _onChangeFlexFactor(int? newFlexFactor, VoidCallback markAsDirty) async {
    final group = objectGroup;
    if (group == null) return;
    markAsDirty();
    await group.invokeSetFlexFactor(
      properties.node.valueRef,
      newFlexFactor,
    );
  }

  void _onChangeFlexFit(FlexFit? newFlexFit, VoidCallback markAsDirty) async {
    final group = objectGroup;
    if (group == null) return;
    markAsDirty();
    await group.invokeSetFlexFit(properties.node.valueRef, newFlexFit!);
  }