#3638 workflow run --detach integration test fails under load: the run finishes before the test attaches

integration/workflow_run_detach_test.ts ('workflow run --detach: returns the id of a recorded run that keeps running on the server (swamp-club#1090)', line 153) failed once in verify-build on 2026-10-19 while the host also ran the swamp-uat uat:cli suite: run.attach rejected with "No active run with id ''" (thrown from the socket.onmessage handler at line 146). It passed 3/3 when re-run alone (about 250 ms each). The test appears to assume the detached run is still active when it attaches; under CPU load the run can finish first. Seen on commit 667825b9 (swamp-club#3613 branch), which does not touch workflows or serve. Expected: the test attaches to a run that is held open until it attaches (for example a step that waits on a signal), so it cannot race the run's completion.