feat: add ais.stats widget support

[sarahdayan]

Summary

• Registers the ais.stats widget in the runtime with passthrough transformParams (no serializable params beyond container)
• Adds StatsWidget to the type union in types.ts
• Enables ais.stats in the toolbar with cssClasses support (root, text) using disabledValue: undefined
• Adds stats.test.tsx with 4 field behavior tests (cssClasses toggle on/off/absent/sub-field)
• Adds AI tool tests for add and edit of stats widget (2 new tests)

Note

Partial prop support, only serializable props handled for now.

Note

This widget is index-dependent, meaning it will render differently (nested) when the index PRs are merged.

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