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/extensions_manager_test.dart

@@ -175,9 +175,9 @@ void main() {
       final s2 = f.services.events.on<ExtensionDeactivated>().listen((e) => deactivated.add(e.id));
       f.services.extensions.register(_Ext(id: 'e'));
       await f.services.extensions.activateAll();
-      await Future<void>.delayed(Duration.zero);
+      await pumpEventQueue();
       await f.services.extensions.deactivate('e');
-      await Future<void>.delayed(Duration.zero);
+      await pumpEventQueue();
       expect(activated, ['e']);
       expect(deactivated, ['e']);
       await s1.cancel();