msglist [nfc]: Assert fetched when updating end markers

This already holds for the existing callers.  Updating end markers
should only happen after the initial fetch.  During the initial fetch,
we have a separate loading indicator and no end markers.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/model/message_list.dart

@@ -288,6 +288,7 @@ mixin _MessageSequence {
 
   /// Update [items] to include markers at start and end as appropriate.
   void _updateEndMarkers() {
+    assert(fetched);
     assert(!(haveOldest && fetchingOlder));
     final startMarker = switch ((fetchingOlder, haveOldest)) {
       (true, _) => const MessageListLoadingItem(MessageListDirection.older),