input: Write a python function to convert a string to an integer and check the input type.
  assert convert_str_to_int("123") == 123
  assert convert_str_to_int("456") == 456
  assert convert_str_to_int("789") == 789
output: def convert_str_to_int(s):
      if not isinstance(s, str):
          raise ValueError("Input must be a string")
      return int(s)