[checkbox group] Fix `onCheckedChange` not running when parent checkbox is present

[mj12albert]

Repro: https://codesandbox.io/p/sandbox/runtime-moon-m4snqz

In this case although checking the parent also changes the checked state of the child, the child checkbox's change handler does not run

<CheckboxGroup>
  <Checkbox parent onCheckedChange={handleParentChange} />
  <Checkbox onCheckedChange={handleChildChange} />
</CheckboxGroup>

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[mj12albert]

Currently onCheckedChange doesn't run when the controlled value changes externally, and Checkbox.Root itself can't differentiate between a external change caused by a parent or this:

<button onClick={() => setChecked(!checked)}>external</button>

<Checkbox.Root
  checked={checked}
  onCheckedChange={(nextChecked) => {
    console.log('checked change');
    setChecked(nextChecked);
  }}
/>

Another thing is if a child checkbox's onCheckedChange is caused by a parent checkbox change, there's no relevant event to pass to the 2nd arg of onCheckedChange. It would need to be undefined for this scenario, or provide the event from the parent checkbox which doesn't seem right

UPDATE: For consistency with other components controlled behavior, checking the parent should not fire the child's onCheckedChange