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/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.")