Follow-up on @Dilligaf371's PR #339 fix. Adds a regression test that
spawns a subprocess which consumes stdin without ever writing to
stdout, then issues `send_notification` from a worker thread. If the
override is missing, the thread blocks forever on `proc.stdout.readline()`
and the 2-second join timeout fires.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>