Don't sleep in ability.Activate() when breathing fire (#77673)

## About The Pull Request

The global proc `dragon_fire_line` sleeps, and was being called inside
`ability.Activate`
This made the CI fail on a different PR because the mob stopped existing
between activating the ability and triggering its cooldown, which should
not regularly be possible.

The ice whelp refactor author noticed this when using it in a loop but I
guess didn't think about the other implications 😅

## Why It's Good For The Game

Fixes a bug.

## Changelog

Not player facing

code/modules/mob/living/basic/icemoon/ice_whelp/ice_whelp_abilities.dm

@@ -12,7 +12,8 @@
 /datum/action/cooldown/mob_cooldown/ice_breath/Activate(atom/target_atom)
 	var/turf/target_fire_turf = get_ranged_target_turf_direct(owner, target_atom, fire_range)
 	var/list/burn_turfs = get_line(owner, target_fire_turf) - get_turf(owner)
-	dragon_fire_line(owner, burn_turfs, frozen = TRUE)
+	// This proc sleeps
+	INVOKE_ASYNC(GLOBAL_PROC, GLOBAL_PROC_REF(dragon_fire_line), owner, /* burn_turfs = */ burn_turfs, /* frozen = */ TRUE)
 	StartCooldown()
 	return TRUE