refactor: DateTimeTool 注入为单例依赖

为 DateTimeTool 实现 ISingletonDependency，统一生命周期管理，便于依赖注入使用

Yi.Abp.Net8/module/ai-hub/Yi.Framework.AiHub.Domain/Mcp/DateTimeTool.cs

@@ -1,10 +1,11 @@
 using System.ComponentModel;
+using Volo.Abp.DependencyInjection;
 using Yi.Framework.AiHub.Domain.Shared.Attributes;
 
 namespace Yi.Framework.AiHub.Domain.Mcp;
 
 [YiAgentTool]
-public class DateTimeTool
+public class DateTimeTool:ISingletonDependency
 {
     [YiAgentTool("时间"), DisplayName("DateTime"), Description("获取当前日期与时间")]
     public DateTime DateTime()