Keep the persistent child SIGKILL test off the runner's process group

Summary

• The persistent child SIGKILL test in gateway-stream-lifecycle.test.ts writes its pid file before the marker it waits for, so it no longer reads a missing or empty pid file.
• The test's cleanup signals only a recorded pid above 1 that is not the runner, so an empty pid file can no longer send SIGKILL to pid 0 and kill the runner's own process group.
• Cleanup skips the recorded pids once they have exited, so it never signals a pid the system has given to another process.