This PR improves the reliability of `raw-exec-process-group.test`,
addressing [#415](https://github.com/openai/codex/issues/415)
Before: The test would fail sporadically in CI because it checked for
process termination immediately after abort, without accounting for the
time it takes for processes to fully terminate.
Now: We've added a robust `ensureProcessGone` helper that:
- Polls the process status with a 500ms timeout
- Retries every 50ms if the process is still alive
- Provides clear error messages if termination takes too long
We now wait for the child process to fully exit after sending abort
signals, instead of assuming instant death, fixing flakiness caused by
asynchronous process termination.
Changes:
- Added `ensureProcessGone` helper function with retry logic
- Improved error handling and timeout management
See [this bash
demo](https://gist.github.com/jdocherty/a84dbca2fbf7b47e5f95c87a07034ae8)
for a minimal reproduction of why process death is asynchronous and why
the test needs to retry after aborting.
