msglist test: Pin the clock in the date-separator test, to avoid DST flakes

This flake hasn't caused any failures I'm aware of; it's much rarer
than the one fixed by PR zulip#2454, the flake that got our suite removed
from Flutter's customer tests registry. But it's the same class of
bug, discovered while fixing that other one, and the fix is simple.

With PR zulip#2454 sent, this completes my Claude-powered sweep of
DST-related test flakes.

Verified with the clock at 2025-11-02 23:30 America/Los_Angeles: the
test fails there before this change and passes after.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: chrisbobbe

test/model/message_list_test.dart

@@ -3681,7 +3681,11 @@ void main() {
   });
 
   group('findItemWithMessageId', () {
-    test('has MessageListDateSeparatorItem with null message ID', () => awaitFakeAsync((async) async {
+    test('has MessageListDateSeparatorItem with null message ID', () => awaitFakeAsync(
+        // Pin the clock, to noon, so the message is definitely dated on the
+        // previous calendar day. (This avoids flakes involving a 25-hour day
+        // when clocks fall back for Daylight Saving Time.)
+        initialTime: DateTime(2035, 8, 21, 12), (async) async {
       final stream = eg.stream();
       final message = eg.streamMessage(stream: stream, topic: 'topic',
         timestamp: eg.utcTimestamp(clock.daysAgo(1)));