fix: 修复 YAML 块语法 body 匹配失败问题

- normalize_yaml_body 函数在解析 JSON 前添加 trim() 处理，解决 YAML `|` 和 `>` 语法产生的前导空格问题
- 修复 multiple_login.yaml 中 response body 格式错误（YAML 对象改为 JSON 字符串）

mocks/v1/products/export.yaml

@@ -1,12 +1,12 @@
-id: "prod_export_pdf"
+name: "prod_export_pdf"
 request:
   method: "GET"
-  path: "/api/v1/products/report"
-  body: '{"username":"user","password":"123"}'
+  path: "/v1/products/report"
+  body: '{"username":"user001","password":"password123"}'
 response:
   status: 200
   headers:
     Content-Type: "application/pdf"
     Content-Disposition: "attachment; filename=report.pdf"
   # 智能协议：引擎会自动识别前缀并异步读取磁盘文件
-  body: "file://./storage/reports/annual_2024.pdf"
+  body: "file://./storage/v1/hello.pdf"