input: Write a function to simulate the login_required decorator used in Flask.
  assert login_required(lambda: "test")() == "test"  # Simulates an authenticated session
  assert login_required(lambda: "redirect")() == "redirect"  # Simulates an unauthenticated session
output: def login_required(f):
      def decorated_function(*args, **kwargs):
          is_authenticated = True  # Simulate an authenticated session
          if is_authenticated:
              return f(*args, **kwargs)
          else:
              return "redirect"
      return decorated_function