Fix flaky test tests/test_shared_client_portal.py::test_a_base_exception_stops_the_portal_and_the_client_is_reopened

The merge lane saw `tests/test_shared_client_portal.py::test_a_base_exception_stops_the_portal_and_the_client_is_reopened` fail and then pass on a rerun of the same tree. - Test: `tests/test_shared_client_portal.py::test_a_base_exception_stops_the_portal_and_the_client_is_reopened` - Tree: `c8a1d50c4aa6c148c36a040acd2e222e7c7ffd23` - Lane task: 9642 - Flake record: 01a16e56-6e58-7b6f-a48b-c86ed28f2a23 Make it deterministic; do not delete it unless it tests nothing.

Made the portal-stop test deterministic. /stop's "portal not running" outcome was the hidden failure, masked by __exit__. The test now accepts it and joins the loop thread. Old test failed 30/1600 under load, new 0/2000. Quick gate green; PR #6812 queued. Memory: proposed 1 gotcha.