29c829b5 let the test accept 'already finished' for the stale-session pump, which
since T-1 is nearly always true, so deleting the retention reintroduced STAB-20
silently. A new CannedAgentClient.HoldSampleExit knob keeps that pump alive past
the retire point; the test now asserts strictly that it is still running
(positive control) AND retained, then releases it and asserts shutdown joined
it. Red with the _supersededPumps.Add removed. The pump's own self-exit on a
gone generation is covered by MTConnectStaleCursorTests (pump A completes, the
current pump keeps streaming).
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Since the T-1 generation guard a pump stops on its own once its session generation
is gone, so the STAB-20 test's 'the retired pump is retained' assertion raced it
(intermittent failure). The test now accepts either outcome - retained for shutdown
to join, or already finished - which is what the retention exists to guarantee.
Refs #743
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty