feat(chat): 新增会话管理支持多轮对话

- 引入 KeyboardAiChatSession 实体及对应 Mapper、Service
- 为 KeyboardAiChatMessage 增加 session_id 字段
- ChatServiceImpl 保存消息时绑定会话，支持按用户+角色获取或创建活跃会话
- 保证同一用户同一角色的连续对话归属同一会话，实现多轮上下文管理

src/main/java/com/yolo/keyborad/mapper/KeyboardAiChatSessionMapper.java

@@ -0,0 +1,12 @@
+package com.yolo.keyborad.mapper;
+
+import com.baomidou.mybatisplus.core.mapper.BaseMapper;
+import com.yolo.keyborad.model.entity.KeyboardAiChatSession;
+
+/*
+* @author: ziin
+* @date: 2026/1/28 16:20
+*/
+    
+public interface KeyboardAiChatSessionMapper extends BaseMapper<KeyboardAiChatSession> {
+}