MM-43939: fixes LastReplyAt when deleting the last reply (#20615)

* MM-43939: fixes lastreplyat when deleting the last reply

Currently we are not updating the Threads.LastReplyAt when the last
reply gets deleted. This can lead to threads appearing unread when
actually there is no unread thread.

This commit updates the value of Threads.LastReplyAt when a reply gets
deleted, to the most recent post's timestamp in the thread.

* Updates ReplyCount to current value on post delete

* Addresses review comments

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

store/sqlstore/post_store.go

@@ -2840,8 +2840,23 @@ func (s *SqlPostStore) updateThreadAfterReplyDeletion(transaction *sqlxTxWrapper
 			}
 		}
 
+		lastReplyAtSubquery := sq.Select("COALESCE(MAX(CreateAt), 0)").
+			From("Posts").
+			Where(sq.Eq{
+				"RootId":   rootId,
+				"DeleteAt": 0,
+			})
+
+		lastReplyCountSubquery := sq.Select("Count(*)").
+			From("Posts").
+			Where(sq.Eq{
+				"RootId":   rootId,
+				"DeleteAt": 0,
+			})
+
 		updateQueryString, updateArgs, err := updateQuery.
-			Set("ReplyCount", sq.Expr("ReplyCount - 1")).
+			Set("LastReplyAt", lastReplyAtSubquery).
+			Set("ReplyCount", lastReplyCountSubquery).
 			Where(sq.And{
 				sq.Eq{"PostId": rootId},
 				sq.Gt{"ReplyCount": 0},