msglist: Use store.senderDisplayName for sender row

[chris: added tests]

Co-authored-by: Chris Bobbe <[email protected]>

Author: PIG208

lib/widgets/message_list.dart

@@ -1437,7 +1437,7 @@ class _SenderRow extends StatelessWidget {
                     userId: message.senderId),
                   const SizedBox(width: 8),
                   Flexible(
-                    child: Text(message.senderFullName, // TODO(#716): use `store.senderDisplayName`
+                    child: Text(store.senderDisplayName(message),
                       style: TextStyle(
                         fontSize: 18,
                         height: (22 / 18),

test/widgets/message_list_test.dart

@@ -1412,6 +1412,30 @@ void main() {
   });
 
   group('MessageWithPossibleSender', () {
+    testWidgets('known user', (tester) async {
+      final user = eg.user(fullName: 'Old Name');
+      await setupMessageListPage(tester,
+        messages: [eg.streamMessage(sender: user)],
+        users: [user]);
+
+      check(find.widgetWithText(MessageWithPossibleSender, 'Old Name')).findsOne();
+
+      // If the user's name changes, the sender row should update.
+      await store.handleEvent(RealmUserUpdateEvent(id: 1,
+        userId: user.userId, fullName: 'New Name'));
+      await tester.pump();
+      check(find.widgetWithText(MessageWithPossibleSender, 'New Name')).findsOne();
+    });
+
+    testWidgets('unknown user', (tester) async {
+      final user = eg.user(fullName: 'Some User');
+      await setupMessageListPage(tester, messages: [eg.streamMessage(sender: user)]);
+      check(store.getUser(user.userId)).isNull();
+
+      // The sender row should fall back to the name in the message.
+      check(find.widgetWithText(MessageWithPossibleSender, 'Some User')).findsOne();
+    });
+
     testWidgets('Updates avatar on RealmUserUpdateEvent', (tester) async {
       addTearDown(testBinding.reset);