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/health.json

@@ -0,0 +1,14 @@
+{
+  "name": "health_check",
+  "request": {
+    "method": "GET",
+    "path": "/v1/health"
+  },
+  "response": {
+    "status": 200,
+    "headers": {
+      "Content-Type": "application/json"
+    },
+    "body": "{\"status\":\"healthy\",\"version\":\"2.0.0\",\"timestamp\":\"2026-03-27T10:00:00Z\"}"
+  }
+}