新增「删除本轮对话」：在聊天中可删除任意一轮的用户消息与助手回复（含该轮关联的过程详情），用于去掉无效或错误内容、减轻上下文干扰。删除后会同步清空该会话保存的 ReAct 上下文快照（last_react_input / last_react_output），保证下一轮模型输入仅基于剩余消息记录，与上下文压缩逻辑一致。流式对话下在用户/助手消息落库后会通过 SSE 下发消息 ID，删除入口可即时显示而无需刷新页面。
Delete this conversation turn (new): Remove a full turn—user message, assistant reply, and linked process details—to drop noise or mistakes and reduce context clutter. The session’s saved ReAct snapshot (last_react_input / last_react_output) is cleared so the next model call rebuilds history only from the remaining stored messages, staying consistent with context compression. During streaming, message IDs are pushed over SSE after persistence, so the delete control appears without a full page reload.