[Bug] Goal cancellation not working - causes infinite loop in agentic execution

**Bug Description**
goal isnt being properly cancellled and spirals claude into an infinite loop

**Environment Info**
- Platform: darwin
- Terminal: xterm-256color
- Version: 2.1.269
- Feedback ID: 188ccbff-256b-452d-888e-5abe2dee15f1

**Errors**
```json
[{"error":"Error: 500 {\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"},\"request_id\":\"req_011Cf183GuTbwTPqrvVJmbuQ\"}\n at generate (/$bunfs/root/chunk-2n2dmkkb.js:11:14493)\n at makeRequest (/$bunfs/root/chunk-2n2dmkkb.js:51:8033)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-09-13T08:28:21.956Z"},{"error":"Error: 500 {\"type\":\"error\",\"error\":{\"type\":\"api_error\",\"message\":\"Internal server error\"},\"request_id\":\"req_011Cf26a2Zdzb8VdavR8eSNA\"}\n at generate (/$bunfs/root/chunk-2n2dmkkb.js:11:14493)\n at makeRequest (/$bunfs/root/chunk-2n2dmkkb.js:51:8033)\n at processTicksAndRejections (native:7:39)","timestamp":"2026-09-13T20:49:42.471Z"}]
```

No source file or test is identified in the report. Reproduce the cancellation scenario on darwin with Claude Code 2.1.269, then trace the goal-cancellation behavior around the repeated API 500 errors. Done means cancelling a goal exits cleanly instead of continuing in an infinite loop, with a regression test covering the behavior.

Written by the indexing model from the issue text.