refactor: change loadModel parameter from string to string array

- Update loadModel signature: url: string -> urls: string[]
- Update validation logic for array parameter
- Update all demo files to pass array instead of single string

demo-vue/src/App.vue

@@ -331,7 +331,7 @@ const loadModel = () => {
     // 加载模型文件（从 model 目录）
     const modelUrl = 'https://lyz-1259524260.cos.ap-guangzhou.myqcloud.com/iflow/models/8634e556-a94e-4ba7-be3e-2ea1507cced5/';
 
-    engine.value.engine.loadModel(modelUrl, {
+    engine.value.engine.loadModel([modelUrl], {
       position: [0, 0, 0],    // 初始位置
       rotation: [0, 0, 0],       // 初始旋转
       scale: [1, 1, 1]         // 初始缩放
@@ -361,7 +361,7 @@ const switchModel = () => {
   }
 
   try {
-    engine.value.engine.loadModel(newUrl.trim(), {
+    engine.value.engine.loadModel([newUrl.trim()], {
       position: [0, 0, 0],
       rotation: [0, 0, 0],
       scale: [1, 1, 1]