Update prefix_sum.py

[pranjaykumar926]

Index Validation for get_sum

Raises ValueError if start or end is out of range or start > end. Handles cases where the array is empty.
✅ Empty Array Support

If an empty array is passed, get_sum raises an appropriate error instead of failing unexpectedly. ✅ Optimized contains_sum Initialization

Initializes sums with {0} for efficient subarray sum checking.

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[pranjaykumar926]

This ensures robustness while maintaining O(1) sum queries and O(n) target sum checks.