fix: 修复对话会话管理问题

- 添加 serviceTier 调试日志
- abortCurrentSession 时清空会话引用，确保下次创建新会话

src/services/dialogService.ts

@@ -369,6 +369,8 @@ export class DialogSession {
     const knowledgeData = await this.loadKnowledgeData();
     console.log('[DialogSession] knowledgeData 加载结果:', knowledgeData ? `${knowledgeData.length} 字符` : 'null');
 
+    console.log('[DialogSession] serviceTier 参数:', serviceTier, '-> 使用:', serviceTier || config.serviceTier);
+
     const request: DialogRequest = {
       taskId: this.taskId,
       message,
@@ -908,6 +910,7 @@ class DialogManager {
    */
   abortCurrentSession(): void {
     this.currentSession?.abort();
+    this.currentSession = null;  // 清空会话，确保下次创建新会话
   }
 }