[MM-60603] Don't follow threads when marking them as read on focus (#28263)

* [MM-60603] Don't follow threads when marking them as read on focus

* Fix tests

* Fix lint

* Fix the original bug in the API call

server/channels/api4/user.go

@@ -3344,6 +3344,14 @@ func setUnreadThreadByPostId(c *Context, w http.ResponseWriter, r *http.Request)
 		return
 	}
 
+	// We want to make sure the thread is followed when marking as unread
+	// https://mattermost.atlassian.net/browse/MM-36430
+	err := c.App.UpdateThreadFollowForUser(c.Params.UserId, c.Params.TeamId, c.Params.ThreadId, true)
+	if err != nil {
+		c.Err = err
+		return
+	}
+
 	thread, err := c.App.UpdateThreadReadForUserByPost(c.AppContext, c.AppContext.Session().Id, c.Params.UserId, c.Params.TeamId, c.Params.ThreadId, c.Params.PostId)
 	if err != nil {
 		c.Err = err