Gate hardening for #5198: the new stream-owner map leaked entries on three
paths that bypass the teardown finally — worker early-return when the stream
was cancelled before startup (streaming.py, gateway_chat.py) and the direct
ACTIVE_RUNS stale-zombie prune (routes.py). Each now calls unregister_stream_owner.
Adds a regression test proving the early-return path no longer leaks.
Co-authored-by: starship-s <starship-s@users.noreply.github.com>
