Largest Square Subgrid Under a Sum Limit

You are given a two-dimensional integer array grid and an integer maxSum.

For a side length k, consider every contiguous k x k square subgrid. The side length is valid when the sum of every such square is at most maxSum.

Return the maximum valid side length.

Source note: The report clearly described the array, the sum limit, and the requirement that every square of the chosen size satisfy the limit. This version is a close reconstruction of that core task; the original example, function signature, and numeric limits were not included.