Add Percentage Calculator with Input Validation

[abhiram-kadarla]

📌 Overview
This pull request introduces a new utility module percentage_calculator.py to the Student Utility Library.
The module provides a reusable function to calculate percentage based on total marks and obtained marks, including proper input validation and formatted output.

✅ Features Implemented

1. Calculates percentage using the formula:
   (obtained_marks / total_marks) × 100
2. Rounds the result to 2 decimal places
3. Validates input values:
4. Raises ValueError if total marks are less than or equal to zero
5. Raises ValueError if obtained marks are negative
6. Includes a test execution block using if name == "main"

Example
Input:
Total Marks: 500
Obtained Marks: 420
Output:
Percentage: 84.00

FILE ADDED:
percentage_calculator.py.

Purpose of This Change
This utility enhances the Student Utility Library by providing a reliable and reusable percentage calculation function. It can be integrated into grading systems, academic tools, or other student-related modules in the future.

Testing:

The functionality was tested locally using sample inputs and verified for:
Correct percentage calculation
Proper rounding
Error handling for invalid inputs

[muskankhedia]

Please implement for an existing function.