Sum of Digits

In this lab you need to implement a function named sum_of_digits in Python. This function takes a single integer parameter and calculates the sum of its digits. The process repeats on the sum until a single-digit number is obtained. The function should then return this single-digit result.

For example:

• If the input is 29, the function calculates 2 + 9 = 11, then repeats the process to get 1 + 1 = 2. Thus, the output is 2.
• For 12345, the sum is 1 + 2 + 3 + 4 + 5 = 15, then 1 + 5 = 6. Therefore, the output is 6.

The function should handle inputs ranging from single-digit numbers to multi-digit numbers, including edge cases like 0. Your implementation will be tested against a series of challenges to validate its correctness under various scenarios.