feat: add Bell Numbers algorithm using Aitken's Array (#7219)

* feat: added Bell Numbers algorithm using Aitken's Array

* style: applied clang-format fixes

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

@@ -0,0 +1,53 @@
+package com.thealgorithms.maths;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import org.junit.jupiter.api.Test;
+
+class BellNumbersTest {
+
+    @Test
+    void testStandardCases() {
+        // Base cases and small numbers
+        assertEquals(1, BellNumbers.compute(0));
+        assertEquals(1, BellNumbers.compute(1));
+        assertEquals(2, BellNumbers.compute(2));
+        assertEquals(5, BellNumbers.compute(3));
+        assertEquals(15, BellNumbers.compute(4));
+        assertEquals(52, BellNumbers.compute(5));
+    }
+
+    @Test
+    void testMediumNumber() {
+        // B10 = 115,975
+        assertEquals(115975, BellNumbers.compute(10));
+        // B15 = 1,382,958,545
+        assertEquals(1382958545L, BellNumbers.compute(15));
+    }
+
+    @Test
+    void testLargeNumber() {
+        // B20 = 51,724,158,235,372
+        // We use the 'L' suffix to tell Java this is a long literal
+        assertEquals(51724158235372L, BellNumbers.compute(20));
+    }
+
+    @Test
+    void testMaxLongCapacity() {
+        // B25 is the largest Bell number that fits in a Java long (signed 64-bit)
+        // B25 = 4,638,590,332,229,999,353
+        assertEquals(4638590332229999353L, BellNumbers.compute(25));
+    }
+
+    @Test
+    void testNegativeInput() {
+        assertThrows(IllegalArgumentException.class, () -> BellNumbers.compute(-1));
+    }
+
+    @Test
+    void testOverflowProtection() {
+        // We expect an exception if the user asks for the impossible
+        assertThrows(IllegalArgumentException.class, () -> BellNumbers.compute(26));
+    }
+}