WhenTheSessionEnds_ItsForwardsSayTheyWentWithIt failed once under a full run. Ended clears IsLive
and then tells each forward its session is over, in one call. The app's dispatcher never splits that,
but the harness's post runs it on the fake shell's thread, so the test's poll on !IsLive could return
between the two and find the forwards still listening.
The test now also waits until every forward says "Stopped with the session.", the last thing
SessionEnded sets. A 300 ms sleep injected between the two steps fails the old test and passes the new
one; the full App.Tests suite passed three runs in a row.