Added HappyNumber algorithm in maths section (#6571)

* Added HappyNumber algorithm in maths section

* Fix Checkstyle: remove trailing spaces in HappyNumber

* Fix formatting: remove trailing spaces and apply clang-format

* Update HappyNumberTest.java

* Removed old HappyNumbersSeq.java as replaced by new HappyNumber.java

src/test/java/com/thealgorithms/maths/HappyNumberTest.java

@@ -0,0 +1,32 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertFalse;
+import static org.junit.jupiter.api.Assertions.assertTrue;
+
+import org.junit.jupiter.api.Test;
+
+public class HappyNumberTest {
+
+    @Test
+    void testHappyNumbers() {
+        // Known happy numbers
+        assertTrue(HappyNumber.isHappy(1));
+        assertTrue(HappyNumber.isHappy(7));
+        assertTrue(HappyNumber.isHappy(19));
+        assertTrue(HappyNumber.isHappy(100));
+    }
+
+    @Test
+    void testUnhappyNumbers() {
+        // Known unhappy numbers
+        assertFalse(HappyNumber.isHappy(2));
+        assertFalse(HappyNumber.isHappy(4));
+        assertFalse(HappyNumber.isHappy(20));
+    }
+
+    @Test
+    void testLargeNumber() {
+        // Just to check behavior with larger input
+        assertTrue(HappyNumber.isHappy(1000000)); // reduces to 1 eventually
+    }
+}