input: Write a function named `calculate_sum_of_digits` that takes an integer as input and returns the sum of its digits. The function should convert the integer to a string, iterate through each character, convert it back to an integer, and sum these integers.
  
  assert calculate_sum_of_digits(6342) == 15
  assert calculate_sum_of_digits(12345) == 15
  assert calculate_sum_of_digits(0) == 0
  assert calculate_sum_of_digits(999) == 27
  assert calculate_sum_of_digits(10001) == 2
output: def calculate_sum_of_digits(n):
      return sum(int(digit) for digit in str(n))