docs: update node name from generate_title to title_generator_node

Updated references to the title generation node in both documentation and code:
- Changed node name from `generate_title` to `title_generator_node` in API design documentation
- Updated filtering logic in agent service to exclude `title_generator_node` from LLM call statistics, token counting, and frontend streaming events
- Maintains same functionality while using more descriptive node name for clarity

docs/gns3-copilot/ai-chat-api-design.md

@@ -221,7 +221,7 @@ await conn.execute("CREATE INDEX IF NOT EXISTS idx_pinned_updated ON chat_sessio
 
 ### Title 自动同步
 
-会话标题由 `generate_title` 节点自动生成，保存在 LangGraph checkpoint 的 `conversation_title` 字段中。
+会话标题由 `title_generator_node` 节点自动生成，保存在 LangGraph checkpoint 的 `conversation_title` 字段中。
 
 **同步机制**：
 1. 流式 Chat 完成后，从 checkpoint 读取最终 state