unreads: Fix false positives in a debug-only warning

We don't expect UpdateMessageEvent to signal a change in a message's
read state; that's what UpdateMessageFlagsEvent is for. Our unreads
model does process `UpdateMessageEvent`s, though, to apply changes
in a message's mentioned state.

Also, we expect the client to be synced with the server for all
messages' read state, assuming !oldUnreadsMissing.

Where we process `UpdateMessageEvent`s, we have a `debugLog` line to
alert us if one of these expectations isn't being met. It's been
giving false positives when we get an UpdateMessageEvent for a bulk
message move where some of the moved messages are read and some
aren't. Since the server only intends the event's `flags` to apply
to the event's `messageId` (one message), we should only be checking
that message, not all the messages in the event's `messageIds`.
From the API doc on `flags`:

https://zulip.com/api/get-events#update_message
> The user's personal message flags for the message with ID
> message_id following the edit.

Author: chrisbobbe

lib/model/unreads.dart

@@ -253,14 +253,13 @@ class Unreads extends ChangeNotifier {
     //   https://chat.zulip.org/#narrow/stream/378-api-design/topic/mark-as-read.20events.20with.20message.20moves.3F/near/1639957
     final bool isRead = event.flags.contains(MessageFlag.read);
     assert(() {
-      if (!oldUnreadsMissing && !event.messageIds.every((messageId) {
-        final isUnreadLocally = _slowIsPresentInDms(messageId) || _slowIsPresentInStreams(messageId);
-        return isUnreadLocally == !isRead;
-      })) {
+      final isUnreadLocally = _slowIsPresentInDms(messageId) || _slowIsPresentInStreams(messageId);
+      final isUnreadInEvent = !isRead;
+      if (!oldUnreadsMissing && isUnreadLocally != isUnreadInEvent) {
         // If this happens, then either:
-        // - the server and client have been out of sync about a message's
+        // - the server and client have been out of sync about the message's
         //   unread state since before this event, or
-        // - this event was unexpectedly used to announce a change in a
+        // - this event was unexpectedly used to announce a change in the
         //   message's 'read' flag.
         debugLog('Unreads warning: got surprising UpdateMessageEvent');
       }