♻️ refactor(build): improve library output filename formatting

- 修改fileName生成逻辑，使用显式条件判断替代三元表达式
- 为umd格式添加明确的.js扩展名，确保输出文件命名一致性
- 保留默认格式的回退处理，增强代码健壮性

packages/stt-sdk-core/vite.config.ts

@@ -7,7 +7,12 @@ export default defineConfig({
     lib: {
       entry: resolve(__dirname, 'src/index.ts'),
       name: 'SttSdkCore',
-      fileName: (format) => `index.${format === 'es' ? 'js' : 'cjs'}`,
+      fileName: (format) => {
+        if (format === 'es') return 'index.js'
+        if (format === 'cjs') return 'index.cjs'
+        if (format === 'umd') return 'index.umd.js'
+        return `index.${format}.js`
+      },
       formats: ['es', 'cjs', 'umd'],
     },
     rollupOptions: {