test: deflake the suite — pumpEventQueue instead of Future.delayed(Duration.zero)

~173 `await Future<void>.delayed(Duration.zero)` async-settle waits across 25
test files yield the microtask queue exactly once; when an event→handler chain
needs more than one hop they lose the race under CI's parallel load, so the
failing set varied run to run. Replace with `await pumpEventQueue()` (the
deterministic drain already used elsewhere in the suite); rewired the shared
settle()/tick() helpers in one shot. menu_bar's toggle-close test gets a bounded
extra pump. Verified green under CI=true.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/kernel/src/services_stubs_test.dart

@@ -283,7 +283,7 @@ void main() {
       addTearDown(n.dispose);
 
       n.warn('clide CLI not on PATH', title: 'dogfood');
-      await Future<void>.delayed(Duration.zero);
+      await pumpEventQueue();
 
       expect(toasts.entries, hasLength(1));
       expect(toasts.entries.single.message, 'dogfood — clide CLI not on PATH');
@@ -299,7 +299,7 @@ void main() {
 
       n.error('boom');
       n.success('done');
-      await Future<void>.delayed(Duration.zero);
+      await pumpEventQueue();
 
       expect(toasts.entries.map((e) => e.severity), [ToastSeverity.error, ToastSeverity.success]);
     });