MM-35298: Follow thread when added to channel (#19311)

* MM-35298: Follow thread when added to channel

* return better error if thread doesn't exist

* update test for possible race

* use correct comparision operator

api4/channel.go

@@ -1698,6 +1698,14 @@ func addChannelMember(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	if postRootId != "" {
+		err := c.App.UpdateThreadFollowForUserFromChannelAdd(cm.UserId, channel.TeamId, postRootId)
+		if err != nil {
+			c.Err = err
+			return
+		}
+	}
+
 	auditRec.Success()
 	auditRec.AddMeta("add_user_id", cm.UserId)
 	c.LogAudit("name=" + channel.Name + " user_id=" + cm.UserId)