test(Checkbox): assert tab navigation in disabled-not-focusable spec

The "disabled checkbox is not focusable via Tab" test had a misleading
name — it only checked `toBeDisabled()` without exercising Tab. Add the
Tab keypress and assert the checkbox does not receive focus so the test
matches its stated behavior.

Author: DreaminDani

tests/forms/checkbox.spec.ts

@@ -426,6 +426,9 @@ describe('Checkbox Visual Regression', () => {
       });
       const checkbox = page.getByRole('checkbox');
       await expect(checkbox).toBeDisabled();
+      await page.locator('body').click();
+      await page.keyboard.press('Tab');
+      await expect(checkbox).not.toBeFocused();
     });
   });
 });