Add Input Validation for Grade Range

[swaparup36]

The add_grade() method calls validate_grade() but we should also add validation for negative numbers and non-numeric inputs.

Task:

• Enhance the validation in add_grade() method
• Add type checking for the grade parameter
• Raise appropriate exceptions with clear messages

Expected Behavior:

manager.add_grade("Math", -5)    # Should raise ValueError
manager.add_grade("Math", "A+")  # Should raise TypeError

[kryptonn36]

I am working on it