MM-34895: LastViewed should be updated correctly for new thread membership (#17673)

Summary
Update LastViewed correctly for new thread membership

Ticket Link
https://mattermost.atlassian.net/browse/MM-34895

api4/user_test.go

@@ -5766,8 +5766,8 @@ func TestMaintainUnreadRepliesInThread(t *testing.T) {
 	// replying to the thread clears reply count, so it should be 0
 	checkThreadListReplies(t, th, th.Client, th.BasicUser.Id, 0, 1, nil)
 
-	// the other user should have 2 replies
-	checkThreadListReplies(t, th, th.SystemAdminClient, th.SystemAdminUser.Id, 2, 1, nil)
+	// the other user should have 1 reply - the reply from the regular user
+	checkThreadListReplies(t, th, th.SystemAdminClient, th.SystemAdminUser.Id, 1, 1, nil)
 
 	// mark all as read for user
 	resp := th.Client.UpdateThreadsReadForUser(th.BasicUser.Id, th.BasicTeam.Id)