[MM-41993]: fixed counting thread mentions in unread root post (#19874)

* removed appending the root post to the posts list

also changed `UpdateAt` to `CreateAt` in thread_store.go in accordance with kyriakos

* fixed failing test after latest change

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

api4/user_test.go

@@ -6522,8 +6522,8 @@ func TestMarkThreadUnreadMentionCount(t *testing.T) {
 	require.Nil(t, appErr)
 
 	rpost, _ := postAndCheck(t, client, &model.Post{ChannelId: th.BasicChannel.Id, Message: "testMsg @" + th.BasicUser2.Username})
-	reply, _ := postAndCheck(t, client, &model.Post{ChannelId: th.BasicChannel.Id, Message: "testReply1", RootId: rpost.Id})
-	postAndCheck(t, client, &model.Post{ChannelId: th.BasicChannel.Id, Message: "testReply2", RootId: rpost.Id})
+	reply1, _ := postAndCheck(t, client, &model.Post{ChannelId: th.BasicChannel.Id, Message: "testReply1 @" + th.BasicUser2.Username, RootId: rpost.Id})
+	reply2, _ := postAndCheck(t, client, &model.Post{ChannelId: th.BasicChannel.Id, Message: "testReply2", RootId: rpost.Id})
 
 	th.SystemAdminClient.UpdateThreadReadForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, model.GetMillis())
 
@@ -6535,7 +6535,12 @@ func TestMarkThreadUnreadMentionCount(t *testing.T) {
 	u, _, _ = th.SystemAdminClient.GetUserThreads(th.BasicUser2.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{})
 	require.EqualValues(t, 1, u.TotalUnreadMentions)
 
-	th.SystemAdminClient.UpdateThreadReadForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, reply.CreateAt)
+	th.SystemAdminClient.UpdateThreadReadForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, reply1.CreateAt)
+
+	u, _, _ = th.SystemAdminClient.GetUserThreads(th.BasicUser2.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{})
+	require.EqualValues(t, 1, u.TotalUnreadMentions)
+
+	th.SystemAdminClient.UpdateThreadReadForUser(th.BasicUser2.Id, th.BasicTeam.Id, rpost.Id, reply2.CreateAt)
 
 	u, _, _ = th.SystemAdminClient.GetUserThreads(th.BasicUser2.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{})
 	require.EqualValues(t, 0, u.TotalUnreadMentions)