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/index.html

@@ -398,7 +398,7 @@
                 // 加载模型文件（从 model 目录）
                 const modelUrl = 'https://lyz-1259524260.cos.ap-guangzhou.myqcloud.com/iflow/models/417664a3-76c8-4d94-9344-1337246a5d4e/';
 
-                engine.engine.loadModel(modelUrl, {
+                engine.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
@@ -455,7 +455,7 @@
             }
 
             try {
-                engine.engine.loadModel(newUrl.trim(), {
+                engine.engine.loadModel([newUrl.trim()], {
                     position: [0, 0, 0],
                     rotation: [0, 0, 0],
                     scale: [1, 1, 1]