Make sumOfDigits long in HarshadNumber.isHarshad (#4973)

fix: make `sumOfDigits` `long` in `HarshadNumber.isHarshad`

Author: vil02

src/main/java/com/thealgorithms/maths/HarshadNumber.java

@@ -15,7 +15,7 @@ public static boolean isHarshad(long n) {
         if (n <= 0) return false;
 
         long t = n;
-        int sumOfDigits = 0;
+        long sumOfDigits = 0;
         while (t > 0) {
             sumOfDigits += t % 10;
             t /= 10;