fix(searches): fix boundary safety and null checks in SaddlebackSearch - #7726

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Description

This pull request fixes boundary safety, null handling, visibility, and adds a convenience overload in SaddlebackSearch:

1. Row & Column Boundary Safety: In find(int[][] arr, int row, int col, int key), the check previously only tested row < 0 || col >= arr[row].length. Passing row >= arr.length would trigger an ArrayIndexOutOfBoundsException when evaluating arr[row]. Furthermore, negative col values were not guarded against and would trigger ArrayIndexOutOfBoundsException on arr[row][col]. Added row >= arr.length, col < 0, and arr[row] == null guards before accessing row contents.
2. Null Array Handling: Updated empty check from if (arr.length == 0) to if (arr == null || arr.length == 0) to avoid NullPointerException and consistently throw IllegalArgumentException.
3. Method Visibility: Changed method modifier from package-private static int[] find to public static int[] find.
4. Convenience Entrypoint: Added overloaded public static int[] find(int[][] arr, int key) that initiates search from the bottom-left corner (find(arr, arr.length - 1, 0, key)).
5. Comprehensive Tests: Added unit tests in SaddlebackSearchTest for the convenience method, null arrays, out-of-bounds rows, negative column indices, and matrix with null rows.