[Bug]: Intermittent issue: Polyscope agent does not stop after clicking Stop button

### Platform

macOS

### Operating system version

macOs Tahoe | 26.3.1 (a)

### System architecture

ARM64 (M1, M2, etc)

### PolyScope Version

Version 0.19.0 (0.19.0)

### Bug description

Occasionally, agents in Polyscope continue running for several hours and do not stop when the Stop button is clicked. This does not happen consistently but occurs often enough to disrupt normal usage.

It blocks further operations while the agent is running and forces a full restart of Polyscope to stop the agent.

It also appears that the agent is not performing any work in the background, but the running timer continues to increase.

### Steps to reproduce

_No response_

### Relevant log output

```shell

```

未提供源文件、测试、复现步骤或日志。首先在 macOS Tahoe（ARM64）上的 Polyscope 0.19.0 中复现 Stop 按钮故障，然后跟踪 agent 的运行状态和计时器；当 Stop 能够稳定地结束 agent 且不需要完整重启时，即表示完成。

由索引模型根据 Issue 内容生成。