Fix plane when dead nested mobs merge with weeds (#5180)

# About the pull request

This PR fixes an issue where dead nested mobs get set to the floor plane
thus disappearing under the wall. The signal for nest deletion is simply
so we can set the plane to the floor plane when dropped to the ground
since it is intentional weeded mobs get set to the floor plane so they
layer under more things.

# Explain why it's good for the game

Visually corrects mobs that died while in a nest.

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>


![image](https://github.com/cmss13-devs/cmss13/assets/76988376/c1370f51-05c8-4f43-b938-0e1ff6bfce2f)

</details>


# Changelog
:cl: Drathek
fix: Fix dead nested mobs disappearing when weeded
/:cl:

code/datums/components/weed_food.dm

@@ -50,6 +50,8 @@
 	var/turf/parent_turf
 	/// The obj that our parent is buckled to and we have registered a signal
 	var/obj/parent_buckle
+	/// A nest our parent is buckled to and we have registered a signal
+	var/obj/structure/bed/nest/parent_nest
 	/// The weeds that we are merging/merged with
 	var/obj/effect/alien/weeds/absorbing_weeds
 	/// The overlay image when merged
@@ -78,6 +80,7 @@
 	QDEL_NULL(weed_appearance)
 	parent_mob = null
 	parent_turf = null
+	parent_buckle = null
 
 /datum/component/weed_food/RegisterWithParent()
 	RegisterSignal(parent_mob, COMSIG_MOVABLE_MOVED, PROC_REF(on_move))
@@ -100,6 +103,8 @@
 		UnregisterSignal(parent_turf, COMSIG_WEEDNODE_GROWTH)
 	if(parent_buckle)
 		UnregisterSignal(parent_buckle, COSMIG_OBJ_AFTER_BUCKLE)
+	if(parent_nest)
+		UnregisterSignal(parent_nest, COMSIG_PARENT_QDELETING)
 
 /// SIGNAL_HANDLER for COMSIG_MOVABLE_MOVED
 /datum/component/weed_food/proc/on_move()
@@ -160,6 +165,15 @@
 		unmerge_with_weeds()
 		return
 
+/// SIGNAL_HANDLER for COMSIG_PARENT_QDELETING of nest
+/datum/component/weed_food/proc/on_nest_deletion()
+	SIGNAL_HANDLER
+
+	if(merged)
+		parent_mob.plane = FLOOR_PLANE
+	UnregisterSignal(parent_nest, COMSIG_PARENT_QDELETING)
+	parent_nest = null
+
 /**
  * Try to start the process to turn into weeds
  * Returns TRUE if started successfully
@@ -243,6 +257,10 @@
 			parent_buckle = parent_mob.buckled
 			RegisterSignal(parent_mob.buckled, COSMIG_OBJ_AFTER_BUCKLE, PROC_REF(on_after_buckle))
 			return FALSE
+		else
+			parent_nest = parent_mob.buckled
+			RegisterSignal(parent_nest, COMSIG_PARENT_QDELETING, PROC_REF(on_nest_deletion))
+
 	if(parent_buckle)
 		UnregisterSignal(parent_buckle, COSMIG_OBJ_AFTER_BUCKLE)
 		parent_buckle = null
@@ -263,7 +281,8 @@
 	ADD_TRAIT(parent_mob, TRAIT_MERGED_WITH_WEEDS, XENO_WEED_TRAIT)
 	parent_mob.anchored = TRUE
 	parent_mob.mouse_opacity = MOUSE_OPACITY_TRANSPARENT
-	parent_mob.plane = FLOOR_PLANE
+	if(!parent_nest)
+		parent_mob.plane = FLOOR_PLANE
 	parent_mob.remove_from_all_mob_huds()
 
 	if(!weed_appearance) // Make a new sprite if we aren't re-merging
@@ -289,6 +308,10 @@
 		UnregisterSignal(absorbing_weeds, COMSIG_PARENT_QDELETING)
 	absorbing_weeds = null
 
+	if(parent_nest)
+		UnregisterSignal(parent_nest, COMSIG_PARENT_QDELETING)
+	parent_nest = null
+
 	REMOVE_TRAIT(parent_mob, TRAIT_MERGED_WITH_WEEDS, XENO_WEED_TRAIT)
 	parent_mob.anchored = FALSE
 	parent_mob.mouse_opacity = MOUSE_OPACITY_ICON