json_prints_only_json_and_never_asks_brokerd failed once under load: the helper wrote the typed
input to a `bxctl chat --json` that never reads stdin and had already exited, and unwrapped the
broken pipe. A broken pipe on that write is now accepted; any other error still fails the test,
and a test whose input must be read checks the output.
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>