fix(群赛马): 将比赛开始消息发送方式从finish改为send

避免使用finish导致命令执行提前结束，确保后续异步比赛任务能正常执行

danding_bot/plugins/group_horse_racing/commands.py

@@ -518,7 +518,7 @@ async def handle_start(bot: Bot, event: Event):
         for horse in room.horses.values():
             horse.state = HorseState.RACING
 
-    await start_cmd.finish("比赛开始！")
+    await start_cmd.send("比赛开始！")
 
     # Run race in background (outside command handler)
     task = asyncio.create_task(run_race_with_settlement(bot, room, scope))