fix: 代码节点缺失引擎时抛出明确异常

- 在 CodeNode 执行前增加运行时引擎空值校验

- 避免空指针失败，错误信息包含 engine 标识

easy-agents-flow/src/main/java/com/easyagents/flow/core/node/CodeNode.java

@@ -57,6 +57,9 @@ public class CodeNode extends BaseNode {
         String newCode = TextTemplate.of(code).formatToString(variables);
 
         CodeRuntimeEngine codeRuntimeEngine = CodeRuntimeEngineManager.getInstance().getCodeRuntimeEngine(this.engine);
+        if (codeRuntimeEngine == null) {
+            throw new IllegalArgumentException("code runtime engine not found: " + this.engine);
+        }
         return codeRuntimeEngine.execute(newCode, this, chain);
     }