feat(test): Add JSON parsing tests and refactor StringHelper utility methods

Author: codeboyzhou

src/main/java/com/github/codeboyzhou/mcp/declarative/util/StringHelper.java

@@ -46,6 +46,6 @@ public static boolean isBlank(String str) {
    * @return the original string if it is not blank, otherwise the default value
    */
   public static String defaultIfBlank(String str, String defaultValue) {
-    return str == null || str.isBlank() ? defaultValue : str;
+    return isBlank(str) ? defaultValue : str;
   }
 }

src/test/java/com/github/codeboyzhou/mcp/declarative/util/JacksonHelperTest.java

@@ -34,9 +34,25 @@ void testToJsonString_shouldSucceed() {
 
   @Test
   void testToJsonString_shouldThrowException() {
-    CircularReference circularRef = new CircularReference();
     assertThrows(
-        McpServerJsonProcessingException.class, () -> JacksonHelper.toJsonString(circularRef));
+        McpServerJsonProcessingException.class,
+        () -> JacksonHelper.toJsonString(new CircularReference()));
+  }
+
+  @Test
+  void testFromJson_shouldSucceed() {
+    String json = "{\"name\":\"test\",\"age\":25}";
+    Person person = JacksonHelper.fromJson(json, Person.class);
+    assertEquals("test", person.name);
+    assertEquals(25, person.age);
+  }
+
+  @Test
+  void testFromJson_shouldThrowException() {
+    String json = "{\"name\":\"test\",\"age\":25}";
+    assertThrows(
+        McpServerJsonProcessingException.class,
+        () -> JacksonHelper.fromJson(json, CircularReference.class));
   }
 
   @Test

src/test/java/com/github/codeboyzhou/mcp/declarative/util/StringHelperTest.java

@@ -11,6 +11,17 @@ void testConstructor_shouldThrowException() {
     assertThrows(UnsupportedOperationException.class, StringHelper::new);
   }
 
+  @Test
+  void testIsBlank_shouldReturnTrueWhenStrIsBlank() {
+    assertTrue(StringHelper.isBlank(StringHelper.EMPTY));
+    assertTrue(StringHelper.isBlank(StringHelper.SPACE));
+  }
+
+  @Test
+  void testIsBlank_shouldReturnFalseWhenStrIsNotBlank() {
+    assertFalse(StringHelper.isBlank("test"));
+  }
+
   @Test
   void testDefaultIfBlank_shouldReturnDefaultValueWhenStrIsNull() {
     assertEquals("default", StringHelper.defaultIfBlank(null, "default"));