Remove global x-model radio flag and add regression tests

[utsavsomaiya]

Summary

• replace the global window.fromModel flag with an explicit internal { fromModel: true } option when syncing x-model values
• thread the new option through bind() -> bindInputValue() for radio handling
• add Vitest coverage for radio behavior with and without model-driven updates

Why

Using a global flag for model synchronization can leak state and cause unintended coupling. This change keeps model context explicit and local.

Testing

• npm run vitest
• npm run build