fix(core): Fix NPE in Toolkit.getTool when tool name is null (agentscope-ai#982)

## AgentScope-Java Version

1.0.11

## Description

* Closes: agentscope-ai#973 
* Fix NPE in Toolkit.getTool when tool name is null

## Checklist

Please check the following items before code is ready to be reviewed.

- [x]  Code has been formatted with `mvn spotless:apply`
- [x]  All tests are passing (`mvn test`)
- [x]  Javadoc comments are complete and follow project conventions
- [x] Related documentation has been updated (e.g. links, examples,
etc.)
- [x]  Code is ready for review

Author: guanxuc

agentscope-core/src/main/java/io/agentscope/core/tool/ToolRegistry.java

@@ -50,6 +50,9 @@ class ToolRegistry {
      * @param registered RegisteredToolFunction wrapper with metadata
      */
     void registerTool(String toolName, AgentTool tool, RegisteredToolFunction registered) {
+        if (toolName == null || toolName.isBlank()) {
+            throw new IllegalArgumentException("Tool name cannot be null or blank");
+        }
         tools.put(toolName, tool);
         registeredTools.put(toolName, registered);
     }
@@ -61,6 +64,9 @@ void registerTool(String toolName, AgentTool tool, RegisteredToolFunction regist
      * @return AgentTool or null if not found
      */
     AgentTool getTool(String name) {
+        if (name == null || name.isBlank()) {
+            return null;
+        }
         return tools.get(name);
     }
 
@@ -71,6 +77,9 @@ AgentTool getTool(String name) {
      * @return RegisteredToolFunction or null if not found
      */
     RegisteredToolFunction getRegisteredTool(String name) {
+        if (name == null || name.isBlank()) {
+            return null;
+        }
         return registeredTools.get(name);
     }
 
@@ -98,6 +107,9 @@ Map<String, RegisteredToolFunction> getAllRegisteredTools() {
      * @param toolName Tool name to remove
      */
     void removeTool(String toolName) {
+        if (toolName == null || toolName.isBlank()) {
+            throw new IllegalArgumentException("Tool name cannot be null or blank");
+        }
         tools.remove(toolName);
         registeredTools.remove(toolName);
     }

agentscope-core/src/test/java/io/agentscope/core/tool/ToolRegistryTest.java

@@ -19,13 +19,15 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertNotSame;
 import static org.junit.jupiter.api.Assertions.assertNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
 import io.agentscope.core.message.ToolResultBlock;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.Set;
 import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Test;
 import reactor.core.publisher.Mono;
 
@@ -297,4 +299,64 @@ void testConcurrentAccess() {
         // Assert
         assertEquals(200, registry.getToolNames().size());
     }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException register null name tool")
+    void testRegisterNullNameTool() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> registry.registerTool(null, mockTool1, registered1));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException register empty name tool")
+    void testRegisterEmptyNameTool() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> registry.registerTool("", mockTool1, registered1));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException register blank name tool")
+    void testRegisterBlankNameTool() {
+        assertThrows(
+                IllegalArgumentException.class,
+                () -> registry.registerTool("  ", mockTool1, registered1));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException remove null name tool")
+    void testRemoveNullNameTool() {
+        assertThrows(IllegalArgumentException.class, () -> registry.removeTool(null));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException remove empty name tool")
+    void testRemoveEmptyNameTool() {
+        assertThrows(IllegalArgumentException.class, () -> registry.removeTool(""));
+    }
+
+    @Test
+    @DisplayName("Should throw IllegalArgumentException remove blank name tool")
+    void testRemoveBlankNameTool() {
+        assertThrows(IllegalArgumentException.class, () -> registry.removeTool("  "));
+    }
+
+    @Test
+    @DisplayName("Should return null get null name tool")
+    void testGetNullNameTool() {
+        assertNull(registry.getTool(null));
+    }
+
+    @Test
+    @DisplayName("Should return null get empty name tool")
+    void testGetEmptyNameTool() {
+        assertNull(registry.getTool(""));
+    }
+
+    @Test
+    @DisplayName("Should return null get blank name tool")
+    void testGetBlankNameTool() {
+        assertNull(registry.getTool("  "));
+    }
 }