Fix timing issue in JSON cookie test by adding poll_for_token call

- Add back poll_for_token() call before accessing DOM elements
- Follows the same pattern as the existing working test
- Ensures app is fully loaded before test execution

Co-Authored-By: masen@reflex.dev <masen@reflex.dev>

Author: devin-ai-integration[bot]

tests/integration/test_client_storage.py

@@ -818,6 +818,8 @@ def set_sub(var: str, value: str):
         input_value_input.send_keys(value)
         set_sub_state_button.click()
 
+    token = poll_for_token()
+
     # Get references to cookie elements
     c1 = driver.find_element(By.ID, "c1")
     c2 = driver.find_element(By.ID, "c2")