refactor(wechat): 移除无用的根路径接口

- 删除了返回 "Hello from Spring backend!" 的根路径接口
- 保留了获取 openid 的核心接口
- 简化了控制器的路由配置
- 减少了不必要的接口暴露

src/main/java/work/baiyun/chronicdiseaseapp/controller/WeChatController.java

@@ -36,10 +36,6 @@ public class WeChatController {
     @Autowired
     private TokenService tokenService;
 
-    @RequestMapping("/")
-    public R<?> hello() {
-        return R.success(200, "Hello from Spring backend!");
-    }
 
     @Operation(summary = "获取 openid", description = "根据小程序 code 获取 openid 并创建/返回用户 token")