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

code/modules/unit_tests/_unit_tests.dm

@@ -11,6 +11,7 @@
 #include "component_tests.dm"
 #include "config_sanity.dm"
 #include "crafting_lists.dm"
+#include "element_tests.dm"
 #include "emotes.dm"
 #include "init_sanity.dm"
 #include "log_format.dm"

code/modules/unit_tests/element_tests.dm

@@ -0,0 +1,4 @@
+/datum/unit_test/bespoke_element/Run()
+	for(var/datum/element/element_type as anything in subtypesof(/datum/element))
+		if(initial(element_type.element_flags) & ELEMENT_BESPOKE && initial(element_type.id_arg_index) == INFINITY)
+			Fail("Element type [element_type] has ELEMENT_BESPOKE and a default id_arg_index.")