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/single_login.yaml

@@ -0,0 +1,29 @@
+# 用户登录 - JSON 格式
+name: "user_login_001"
+request:
+  path: "/v1/auth/login"
+  method: "POST"
+  headers:
+    Content-Type: "application/json"
+    Authorization: "eyJhbGciOiJIUzI1NiIsInR5cCI6"
+    host: "127.0.0.1:8080"
+  body:
+    username: "user001"
+    password: "password123"
+response:
+  status: 200
+  headers:
+    Content-Type: "application/json"
+  body: |
+    {
+      "code": 0,
+      "message": "登录成功",
+      "data": {
+        "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6",
+        "userId": 10001,
+        "username": "user001",
+        "role": "administrator"
+      }
+    }
+settings:
+  delay_ms: 2000 # 模拟真实网络延迟