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/user/avatar.yaml

@@ -0,0 +1,73 @@
+# 上传头像 - Multipart 格式（数组形式，只匹配字段名）
+- name: "user_upload_avatar_001"
+  request:
+    method: "POST"
+    path: "/v1/user/avatar"
+    headers:
+      Content-Type: "multipart/form-data"
+    body:
+      - "avatar1"
+      - "description1"
+  response:
+    status: 200
+    headers:
+      Content-Type: "application/json"
+    body: |
+      {
+        "code": 0,
+        "message": "头像上传成功",
+        "data": {
+          "url": "https://cdn.example.com/v1/avatars/10001.jpg",
+          "size": 204800,
+          "filename": "avatar.jpg"
+        }
+      }
+
+- name: "user_upload_avatar_002"
+  request:
+    method: "POST"
+    path: "/v1/user/avatar"
+    headers:
+      Content-Type: "multipart/form-data"
+    body:
+      avatar2: "avatar"
+      description2: "description"
+  response:
+    status: 200
+    headers:
+      Content-Type: "application/json"
+    body: |
+      {
+        "code": 0,
+        "message": "头像上传成功",
+        "data": {
+          "url": "https://cdn.example.com/v1/avatars/10002.jpg",
+          "size": 204800,
+          "filename": "avatar.jpg"
+        }
+      }
+- name: "user_upload_avatar_003"
+  request:
+    method: "POST"
+    path: "/v1/user/avatar"
+    headers:
+      Content-Type: "multipart/form-data"
+    body: >
+      {
+        "avatar3": "avatar"
+        "description3": "description"
+      }
+  response:
+    status: 200
+    headers:
+      Content-Type: "application/json"
+    body: |
+      {
+        "code": 0,
+        "message": "头像上传成功",
+        "data": {
+          "url": "https://cdn.example.com/v1/avatars/10003.jpg",
+          "size": 204800,
+          "filename": "avatar.jpg"
+        }
+      }