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/builtin/diff/diff_controller_test.dart

@@ -54,7 +54,7 @@ void main() {
     expect(c.focusPath, 'lib/b.dart');
     expect(notified, greaterThan(0));
     // focus() reloads so the latest edits to that file are present.
-    await Future<void>.delayed(Duration.zero);
+    await pumpEventQueue();
     expect(diffCalls, greaterThan(before));
   });
 
@@ -62,13 +62,13 @@ void main() {
     c.focus('lib/gone.dart');
     expect(c.focusPath, 'lib/gone.dart');
     // The reload triggered by focus() returns a list without that file.
-    await Future<void>.delayed(Duration.zero);
+    await pumpEventQueue();
     expect(c.focusPath, isNull);
   });
 
   test('a focus that stays in the diff survives reload', () async {
     c.focus('lib/a.dart');
-    await Future<void>.delayed(Duration.zero);
+    await pumpEventQueue();
     expect(c.focusPath, 'lib/a.dart');
   });
 
@@ -76,7 +76,7 @@ void main() {
     await c.load();
     final before = diffCalls;
     bus.emit(DaemonEvent(subsystem: 'git', kind: 'git.changed', data: const {}, ts: DateTime.now().toUtc()));
-    await Future<void>.delayed(Duration.zero);
+    await pumpEventQueue();
     expect(diffCalls, greaterThan(before));
   });