[security] GitHib Copilot cannot be stopped or canceled

I'm seeing several occasions in which GitHub Copilot starts reasoning or thinking with Sonnet 5, and then cannot be stopped. The "cancel" button in the field is present and enabled, but it does nothing.

This is similar to past issues such as #291713 and #295351 in which the agent could not be stopped. Unlike #284678 this time it's happening on a new prompt (not a resubmitted prompt).

```
Version: 1.128.0 (user setup)
Commit: fc3def6774c76082adf699d366f31a557ce5573f
Date: 2026-07-07T15:14:24-07:00
Electron: 42.5.0
ElectronBuildId: 14525058
Chromium: 148.0.7778.271
Node.js: 24.17.0
V8: 14.8.178.33-electron.0
OS: Windows_NT x64 10.0.26200
```