fixing runtime errors when removing a brain with skillchips. (#88178)

## About The Pull Request
The organs code changes bundled with the organ fishing PR have broken a
few things, while unveiling a bunch of other problems that flew under
the radar. This concern the first category actually. Anyway, I've also
included a unit test to ensure this won't be an inssue in the future.

## Why It's Good For The Game
Fixing some errors.

code/modules/library/skill_learning/skillchip.dm

@@ -97,25 +97,28 @@
  * Arguments:
  * * silent - Boolean. Whether or not an activation message should be shown to the user.
  * * force - Boolean. Whether or not to just force de-activation if it would be prevented for any reason.
+ * * brain_owner - the owner var of the brain is set to null on organ/on_mob_remove(), so we need this if owner is null.
  */
-/obj/item/skillchip/proc/try_deactivate_skillchip(silent = FALSE, force = FALSE)
+/obj/item/skillchip/proc/try_deactivate_skillchip(silent = FALSE, force = FALSE, mob/living/brain_owner)
 	if(!active)
 		return "Skillchip is not active."
 
 	// Should not happen. Holding brain is destroyed and the chip hasn't had its state set appropriately.
 	if(!holding_brain)
-		stack_trace("Skillchip's owner is null or qdeleted brain.")
+		stack_trace("Skillchip doesn't have a holding brain.")
 		return "Skillchip cannot detect viable brain."
 
+	if(!brain_owner)
+		brain_owner = holding_brain.owner
 	// Also should not happen. We're somehow deactivating skillchips in a bodyless brain.
-	if(QDELETED(holding_brain.owner))
+	if(QDELETED(brain_owner))
 		active = FALSE
 		stack_trace("Skillchip's brain has no owner, owner is null or owner qdeleted.")
 		return "Skillchip cannot detect viable body."
 
 	// We have a holding brain, the holding brain has an owner. If we're forcing this, do it hard and fast.
 	if(force)
-		on_deactivate(holding_brain.owner, silent)
+		on_deactivate(brain_owner, silent)
 		return
 
 	// Is the chip still experiencing a cooldown period?
@@ -123,7 +126,7 @@
 		return "Skillchip is still recharging for [COOLDOWN_TIMELEFT(src, chip_cooldown) * 0.1]s"
 
 	// We're good to go. Deactive this chip.
-	on_deactivate(holding_brain.owner, silent)
+	on_deactivate(brain_owner, silent)
 
 /**
  * Called when a skillchip is inserted in a user's brain.