fix(test): kill the a11y suite's pending-timer flake at its root

Finally caught with a creation stack trace: when the last gRPC
stream closes, Http2ClientConnection arms the channel's 5-minute
idleTimeout timer — even on a shut-down connection — so the
test's 1-minute drain never covered it and the framework's
pending-timer invariant tripped whenever the arm landed inside
the test window (frequent while a real hub listens on 50051).

The suite now closes the channel in real-async space at the end
of the body (new @visibleForTesting HubService.debugResetChannel;
shutdown is deliberately not awaited — it wedges on a mid-connect
socket, but cancels its timers synchronously) and pumps past the
idle timeout so the timer fires inside the test. 6 consecutive
full-suite runs + 3 isolated runs green; before, roughly 1 in 3
full runs failed.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

test/a11y_test.dart

@@ -88,6 +88,19 @@ void main() {
       // !timersPending after the test body.
       await tester.pumpWidget(const SizedBox.shrink());
       await tester.pump(const Duration(minutes: 1));
+      // The suite's long-standing pending-timer flake, finally
+      // caught with a creation stack: when the last gRPC stream
+      // closes, Http2ClientConnection._handleActiveStateChanged
+      // arms the channel's 5-minute idleTimeout timer — even on a
+      // shut-down connection — so a 1-minute drain never covered
+      // it. Close the channel in real-async space (lets in-flight
+      // socket callbacks land), then pump PAST the idle timeout so
+      // the timer fires inside the test body.
+      await tester.runAsync(() async {
+        HubService.instance.debugResetChannel();
+        await Future<void>.delayed(const Duration(milliseconds: 100));
+      });
+      await tester.pump(const Duration(minutes: 6));
       expect(
         violations,
         isEmpty,