[Feature]: 多 Agents 协同中的上下文缺失与轮询阻塞问题 | Multi-Agent Collaboration Context Loss and User Polling Blockage

## Summary

当前在多 agents 协同中，当一个 agent 向另一个 agent 分配任务后，两者并未保持在同一个 session 中。子 agent 在回复时有时也会另开一个 session，这导致两个问题：一是 agents 之间的协同 session 缺乏连续性，缺少上下文；二是与用户 channel 通信的 session 无法收到进度通知，导致用户或主agent不得不手动轮询任务进度，从而阻塞 session 的进行。

Currently, in multi-agent collaboration, after one agent assigns a task to another, they do not remain in the same session. The sub-agent sometimes starts a new session when responding, leading to two issues: first, the collaborative session between agents lacks continuity and context; second, the session communicating with the user channel fails to receive progress notifications, forcing the user or master agents to manually poll the task progress, which blocks the session.

## Component(s) Affected

- [x] Core / Backend (app, agents, config, providers, utils, local_models)