Add comprehensive Test to fix coverage issue

Author: shreyash24-tech

src/test/java/com/thealgorithms/datastructures/trees/KthSmallestElementBSTTest.java

@@ -1,17 +1,56 @@
 package com.thealgorithms.datastructures.trees;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
-
 import org.junit.jupiter.api.Test;
 
-public class KthSmallestElementBSTTest {
+class KthSmallestElementInBSTTest {
 
-    @Test
-    void simpleTest() {
+    private BinaryTree.Node createSampleTree() {
+        /*
+                 5
+               /   \
+              3     7
+             / \   / \
+            2   4 6   8
+         */
         BinaryTree.Node root = new BinaryTree.Node(5);
         root.left = new BinaryTree.Node(3);
         root.right = new BinaryTree.Node(7);
+        root.left.left = new BinaryTree.Node(2);
+        root.left.right = new BinaryTree.Node(4);
+        root.right.left = new BinaryTree.Node(6);
+        root.right.right = new BinaryTree.Node(8);
 
-        assertEquals(3, KthSmallestElementInBST.kthSmallest(root, 1));
+        return root;
+    }
+
+    @Test
+    void testSmallestElement() {
+        BinaryTree.Node root = createSampleTree();
+        assertEquals(2, KthSmallestElementInBST.kthSmallest(root, 1));
+    }
+
+    @Test
+    void testRootElement() {
+        BinaryTree.Node root = createSampleTree();
+        assertEquals(5, KthSmallestElementInBST.kthSmallest(root, 4));
+    }
+
+    @Test
+    void testRightSubtreeElement() {
+        BinaryTree.Node root = createSampleTree();
+        assertEquals(8, KthSmallestElementInBST.kthSmallest(root, 7));
+    }
+
+    @Test
+    void testSingleNodeTree() {
+        BinaryTree.Node root = new BinaryTree.Node(10);
+        assertEquals(10, KthSmallestElementInBST.kthSmallest(root, 1));
+    }
+
+    @Test
+    void testInvalidK() {
+        BinaryTree.Node root = createSampleTree();
+        assertEquals(-1, KthSmallestElementInBST.kthSmallest(root, 10));
     }
 }