Fixes runtime with ridable element and adds unit test for bespoke elements (#26111)

* fixes bespoke elements with default indexes, adds unit test for detecting this

* lewc review

code/datums/elements/ridable.dm

@@ -8,7 +8,7 @@
  */
 /datum/element/ridable
 	element_flags = ELEMENT_BESPOKE
-
+	id_arg_index = 2
 	/// The specific riding component subtype we're loading our instructions from, don't leave this as default please!
 	var/riding_component_type = /datum/component/riding
 	/// If we have a xenobio red potion applied to us, we get split off so we can pass our special status onto new riding components

code/datums/elements/shatters_when_thrown.dm

@@ -3,7 +3,7 @@
  */
 /datum/element/shatters_when_thrown
 	element_flags = ELEMENT_BESPOKE
-
+	id_arg_index = 2
 	/// What type of item is spawned as a 'shard' once the shattering happens
 	var/obj/item/shard_type
 	/// How many shards total are made when the thing we're attached to shatters