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/auth/register.yaml

@@ -0,0 +1,26 @@
+# 用户注册 - JSON 格式
+name: "user_register"
+request:
+  method: "POST"
+  path: "/v1/auth/register"
+  headers:
+    Content-Type: "application/json"
+  body:
+    username: "newuser"
+    password: "newpass123"
+    email: "newuser@example.com"
+response:
+  status: 201
+  headers:
+    Content-Type: "application/json"
+  body: |
+    {
+      "code": 0,
+      "message": "注册成功",
+      "data": {
+        "userId": 10002,
+        "username": "newuser",
+        "email": "newuser@example.com",
+        "createdAt": "2026-03-27T10:00:00Z"
+      }
+    }