Added efficient subarray sum code using sliding window approach (O(n))

[VeeruYadav45]

This PR adds an efficient subarray sum implementation using the sliding window approach, improving performance from O(n²) to O(n).

Changes

1-Implemented subarray sum calculation using a single-pass sliding window algorithm.
2-Optimized for better time complexity (O(n)) and lower memory usage.
3-Added relevant test cases to validate correctness and performance.
4-Updated documentation/comments for better readability.

Benefits

1-Significantly faster for large input arrays.
2-Cleaner and more maintainable code structure.
3-Avoids unnecessary nested loops from the previous approach.