Fix carp migration hard delete (#23064)

* Please describe the intent of your changes in a clear fashion.
This PR addresses issue SERVER-PROD-12K by correcting a hard delete
warning related to `/mob/living/simple_animal/hostile/carp` during the
carp migration event.

Previously, in `code/modules/events/carp_migration.dm`, the `end()` proc
was calling `qdel(carp_weakref)` instead of `qdel(fish)`. This meant the
weakref datum was being deleted, but the actual carp mob was left
without a proper `qdel()` call, leading to BYOND's garbage collector
performing a 'hard delete' and logging a warning.

The fix changes `qdel(carp_weakref)` to `qdel(fish)` on line 103,
ensuring the carp mob is correctly soft-deleted when the event ends.

* Please make sure that, in the case of mapping changes, you include
images of these changes in the PR's description.
(No mapping changes in this PR)
* Please make sure to mark your PR as wip or review required by making a
comment with !wip or !review required
* If you include sprites/sounds/... (assets) that you have not created
yourself specify the license and original author below.
* Ensure that you also credit them in the appropriate location /
changelog as specified in the contributor guidelines

### Asset Licenses
The following assets that **have not** been created by myself are
included in this PR:

| Path | Original Author | License |
| --- | --- | --- |
| icons/example.dmi | ExamplePerson (Example Station) | CC0 |
(No new assets included in this PR)

Fixes
[SERVER-PROD-12K](https://aurorastation.sentry.io/issues/7522667244/?seerDrawer=true)

---------

Co-authored-by: VMSolidus <evilexecutive@gmail.com>

code/modules/events/carp_migration.dm

@@ -97,10 +97,10 @@
 	..()
 
 	for (var/datum/weakref/carp_weakref in spawned_carp)
-		var/mob/living/simple_animal/hostile/carp/fish = carp_weakref.resolve()
+		var/mob/fish = carp_weakref.resolve()
 		if (fish && prob(50) && is_type_in_typecache(fish.loc, despawn_turfs))
 			spawned_carp -= carp_weakref
-			qdel(carp_weakref)
+			qdel(fish)
 	spawned_carp?.Cut()
 
 /datum/event/carp_migration/cozmo/start()

code/modules/events/wandering_psirens.dm

@@ -72,7 +72,7 @@
 		var/mob/living/simple_animal/hostile/psiren/psiren = psiren_weakref.resolve()
 		if (psiren && prob(50) && is_type_in_typecache(psiren.loc, despawn_turfs))
 			spawned_psirens -= psiren_weakref
-			qdel(psiren_weakref)
+			qdel(psiren)
 	spawned_psirens?.Cut()
 
 /datum/event/wandering_psirens/overmap