test(mtconnect): a stale-session pump may finish before the restart retires it

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

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.MTConnect.Tests/MTConnectPumpGenerationTests.cs

@@ -70,6 +70,7 @@ public sealed class MTConnectPumpGenerationTests
 
             // A second subscriber arrives mid-restart: it starts a pump on the (still-live) old client.
             await driver.SubscribeAsync(["dev1_pos"], TimeSpan.FromMilliseconds(50), Ct);
+            var staleSessionPump = driver.SampleStreamTask.ShouldNotBeNull();
             await clientA.WaitForSampleCallsAsync(2).WaitAsync(Watchdog, Ct);
             restart.IsCompleted.ShouldBeFalse();
 
@@ -77,9 +78,12 @@ public sealed class MTConnectPumpGenerationTests
             await restart.WaitAsync(Watchdog, Ct);
 
             // The pump retired by the restart is retained so shutdown can join it (it used to be dropped
-            // un-awaited and could write health after the new session was Healthy).
+            // un-awaited and could write health after the new session was Healthy). Since review 2026-10-21 T-1 a
+            // pump also stops by itself once it sees its session generation is gone, so it may already have
+            // finished before the restart got to retire it — either way it is not left running unjoined.
             retired = driver.SupersededPumpTasks;
-            retired.ShouldNotBeEmpty();
+            (retired.Contains(staleSessionPump) || staleSessionPump.IsCompleted).ShouldBeTrue(
+                "the stale-session pump must be retained for shutdown to join, or already be gone");
 
             // The restart installed a new client; the standing subscription must be streaming from IT.
             await clientB.WaitForSampleCallsAsync(1).WaitAsync(Watchdog, Ct);