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.

[srawlins]

Hi @hannah-hyj , I added you as a reviewer as I think this is a pure "flutter best practices" PR with nothing to do with DevTools business logic. I'd love your input.

[kenzieschmoll]

Why an InheritedWidget here? A few reasons I'd avoid it:

• The drilling is one hop. VisualizeFlexChildren is the only widget between FlexLayoutExplorerWidgetState and FlexChildVisualizer, and it already forwards five other params.
• No rebuild-scoping benefit. _buildLayout runs inside AnimatedBuilder(animation: changeController) → LayoutBuilder, so every consumer is rebuilt top-down anyway. updateShouldNotify can't trigger a rebuild that isn't already happening.
• Compile-time guarantee becomes runtime. required this.state was enforced by the compiler; _of(context) is an assert + !. It also means FlexChildVisualizer can't be built or widget-tested without a scope wrapper. Note this replaces the TODO(polina-c, jacob314): consider refactoring to remove ! with a new !.

#2701 is about what we pass, not how. onTap, onDoubleTap, markAsDirty, and properties are app logic that shouldn't be on a State — that's the root cause. These belong on InspectorController, or on a layout-explorer sub-controller that InspectorController owns and disposes. InspectorController already holds _layoutGroups for this feature and LayoutExplorerWidgetState already has widget.inspectorController, so children can reach it without any new plumbing. That removes the state-passing and makes the logic unit-testable — neither of which the scope gets us. It also drops the markAsDirty threading this PR adds through _buildContent → the two dropdown builders → _onChangeFlexFactor/_onChangeFlexFit.