Slime core mob selection fix (#19289)

Marked some mobs that were not meant to be spawned as abstracts.
Updated the reaction of the slime core to not spawn abstract mobs.

Fixes #19276

code/modules/mob/living/simple_animal/aquatic.dm

@@ -1,5 +1,7 @@
 //For things that swim and don't do much else.
 /mob/living/simple_animal/aquatic
+	abstract_type = /mob/living/simple_animal/aquatic
+
 	name = "aquatic animal"
 	desc = DESC_PARENT
 	icon = 'icons/mob/npc/fish.dmi'

code/modules/mob/living/simple_animal/hostile/commanded/commanded.dm

@@ -1,4 +1,6 @@
 /mob/living/simple_animal/hostile/commanded
+	abstract_type = /mob/living/simple_animal/hostile/commanded
+
 	name = "commanded"
 	var/short_name = null
 	stance = COMMANDED_STOP

code/modules/mob/living/simple_animal/hostile/hostile.dm

@@ -1,4 +1,6 @@
 /mob/living/simple_animal/hostile
+	abstract_type = /mob/living/simple_animal/hostile
+
 	faction = "hostile"
 	var/stance = HOSTILE_STANCE_IDLE	//Used to determine behavior
 	var/mob/living/target_mob

code/modules/mob/living/simple_animal/hostile/retaliate/aquatic.dm

@@ -1,5 +1,7 @@
 //For things that swim and don't do much else, but also bite!
 /mob/living/simple_animal/hostile/retaliate/aquatic
+	abstract_type = /mob/living/simple_animal/hostile/retaliate/aquatic
+
 	name = "aquatic animal"
 	desc = DESC_PARENT
 	icon = 'icons/mob/npc/fish.dmi'

code/modules/mob/living/simple_animal/hostile/retaliate/retaliate.dm

@@ -1,4 +1,6 @@
 /mob/living/simple_animal/hostile/retaliate
+	abstract_type = /mob/living/simple_animal/hostile/retaliate
+
 	var/list/enemies = list()
 
 /mob/living/simple_animal/hostile/retaliate/Destroy()