[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

```

No source files, tests, reproduction steps, or logs are provided. Start by reproducing the Stop-button failure in Polyscope 0.19.0 on macOS Tahoe with ARM64, then trace the agent's running state and timer; done means Stop consistently ends the agent without requiring a full restart.

Written by the indexing model from the issue text.