Implement sliding window algorithms for fixed and dynamic sizes with tests

[Kesavan-77]

Sliding Windows Algorithm

• Added maxSumSubarrayFixed function to calculate the maximum sum of a fixed-size subarray.
• Added longestSubarrayWithSumAtMost function to find the longest subarray with a sum less than or equal to a given target.
• Included tests for both functions to ensure correctness and edge case handling.
• Updated documentation to reflect the new implementations.