feat: mock配置迁移至JSON格式并修复body匹配

- 将mock配置从YAML格式迁移到JSON格式
- 修复JSON字符串格式body匹配失败问题
- 添加MCP功能模块
- 更新mock-spec.md规范文档

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mocks/v1/user/avatar_003.json

@@ -0,0 +1,21 @@
+{
+  "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\"}}"
+  }
+}