refactor: 重构聊天模块架构

- 删除废弃的ChatMessageDTO、ChatContext、AbstractChatMessageService等类
- 迁移ChatServiceFactory和IChatMessageService到ruoyi-chat模块
- 重构ChatHandler体系，移除DefaultChatHandler和ChatContextBuilder
- 优化SSE消息处理，新增SseEventDto
- 简化各AI服务提供商实现类代码
- 优化工作流节点消息处理逻辑

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ruoyi-modules/ruoyi-chat/pom.xml

@@ -19,6 +19,11 @@
             <artifactId>ruoyi-common-chat</artifactId>
         </dependency>
 
+        <dependency>
+            <groupId>org.ruoyi</groupId>
+            <artifactId>ruoyi-common-sse</artifactId>
+        </dependency>
+
         <dependency>
             <groupId>org.ruoyi</groupId>
             <artifactId>ruoyi-common-sensitive</artifactId>