testing: improve test coverage ValidParenthesesTest

Author: alxkm

src/test/java/com/thealgorithms/strings/ValidParenthesesTest.java

@@ -1,15 +1,31 @@
 package com.thealgorithms.strings;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import org.junit.jupiter.params.ParameterizedTest;
 import org.junit.jupiter.params.provider.CsvSource;
+import org.junit.jupiter.api.Test;
 
 public class ValidParenthesesTest {
 
     @ParameterizedTest(name = "Input: \"{0}\" → Expected: {1}")
-    @CsvSource({"'()', true", "'()[]{}', true", "'(]', false", "'{[]}', true", "'([{}])', true", "'([)]', false", "'', true", "'(', false", "')', false"})
+    @CsvSource({"'()', true", "'()[]{}', true", "'(]', false", "'{[]}', true", "'([{}])', true", "'([)]', false", "'', true", "'(', false", "')', false", "'{{{{}}}}', true", "'[({})]', true", "'[(])', false", "'[', false", "']', false", "'()()()()', true", "'(()', false", "'())', false", "'{[()()]()}', true"})
     void testIsValid(String input, boolean expected) {
         assertEquals(expected, ValidParentheses.isValid(input));
     }
+
+    @Test
+    void testNullInputThrows() {
+        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () -> ValidParentheses.isValid(null));
+        assertEquals("Input string cannot be null", ex.getMessage());
+    }
+
+    @ParameterizedTest(name = "Input: \"{0}\" → throws IllegalArgumentException")
+    @CsvSource({"'a'", "'()a'", "'[123]'", "'{hello}'", "'( )'", "'\t'", "'\n'", "'@#$%'"})
+    void testInvalidCharactersThrow(String input) {
+        IllegalArgumentException ex = assertThrows(IllegalArgumentException.class, () -> ValidParentheses.isValid(input));
+        assertTrue(ex.getMessage().startsWith("Unexpected character"));
+    }
 }