Properly unreg's turf changed when space openspace is deleted (#76501)

## About The Pull Request

Openspace tracks the turf below for starlight purposes The trouble is if
a turf is replaced by another, all its signal registers still persist

I forgot about this, so we'd get runtimes when a
/turf/open/space/openspace was replaced, and the turf below it changed

## Why It's Good For The Game

Less random runtimes

code/game/turfs/open/openspace.dm

@@ -23,6 +23,8 @@
 // I am so sorry
 /turf/open/openspace/Initialize(mapload) // handle plane and layer here so that they don't cover other obs/turfs in Dream Maker
 	. = ..()
+	if(PERFORM_ALL_TESTS(focus_only/openspace_clear) && !GET_TURF_BELOW(src))
+		stack_trace("[src] was inited as openspace with nothing below it at ([x], [y], [z])")
 	RegisterSignal(src, COMSIG_ATOM_AFTER_SUCCESSFUL_INITIALIZED_ON, PROC_REF(on_atom_created))
 	var/area/our_area = loc
 	if(istype(our_area, /area/space))

code/game/turfs/open/space/space.dm

@@ -251,6 +251,8 @@
 
 /turf/open/space/openspace/Initialize(mapload) // handle plane and layer here so that they don't cover other obs/turfs in Dream Maker
 	. = ..()
+	if(PERFORM_ALL_TESTS(focus_only/openspace_clear) && !GET_TURF_BELOW(src))
+		stack_trace("[src] was inited as openspace with nothing below it at ([x], [y], [z])")
 	icon_state = "pure_white"
 	// We make the assumption that the space plane will never be blacklisted, as an optimization
 	if(SSmapping.max_plane_offset)
@@ -261,6 +263,13 @@
 	. = ..()
 	AddElement(/datum/element/turf_z_transparency)
 
+/turf/open/space/openspace/Destroy()
+	// Signals persist through destroy, GO HOME
+	var/turf/below = GET_TURF_BELOW(src)
+	if(below)
+		UnregisterSignal(below, COMSIG_TURF_CHANGE)
+	return ..()
+
 /turf/open/space/openspace/zAirIn()
 	return TRUE
 

code/modules/unit_tests/create_and_destroy.dm

@@ -101,6 +101,9 @@ GLOBAL_VAR_INIT(running_create_and_destroy, FALSE)
 	ignore += subtypesof(/obj/machinery/airlock_controller)
 	// Always ought to have an associated escape menu. Any references it could possibly hold would need one regardless.
 	ignore += subtypesof(/atom/movable/screen/escape_menu)
+	// Can't spawn openspace above nothing, it'll get pissy at me
+	ignore += typesof(/turf/open/space/openspace)
+	ignore += typesof(/turf/open/openspace)
 
 	var/list/cached_contents = spawn_at.contents.Copy()
 	var/original_turf_type = spawn_at.type

code/modules/unit_tests/focus_only_tests.dm

@@ -36,5 +36,8 @@
 /// Checks for bad icon / icon state setups in cooking crafting menu
 /datum/unit_test/focus_only/bad_cooking_crafting_icons
 
+/// Ensures openspace never spawns on the bottom of a z stack
+/datum/unit_test/focus_only/openspace_clear
+
 /// Checks to ensure that variables expected to exist in a job datum (for config reasons) actually exist
 /datum/unit_test/focus_only/missing_job_datum_variables