Dismounting someone from a Pony (or Leapers) requires shoving and is not guaranteed (#95517)

## About The Pull Request

Ponies can no longer be one-click dismounted like typical mounts

Instead when you shove a pony (or its rider), there is a chance based on
multiple factors (is already staggered, has stamina damage, is wounded)
that you manage to remove the rider

Successfully disarming a rider this way will knock them down for a very
short duration

This save behavior was applied to the Leaper, though as Leapers cannot
be staggered and are significantly larger, the chance of a successful
dismount is far lower

## Why It's Good For The Game

Being able to be spam clicked to lose your mount is kind of unfortunate

However, being completely immune to dismount is also not ideal

So as a middle ground, a little stability minigame makes sense to me -
it gives both sides agency in avoiding or pursuing a dismount. The rider
can keep a distance to recover stability while the attackers can use
weapons (even batons or disablers) to make it easier to remove the
rider.

## Changelog

🆑 Melbert
balance: You can no longer dismount riders from ponies with a single
click. Instead, there is a chance to dismount the rider on shoving the
pony OR the rider, based on multiple factors (such as stamina + physical
damage and stagger)
balance: The Leaper mount also has this "disarm to dismount" mechanic,
though it is significantly more difficult as they are much sturdier
beasts
/🆑

code/modules/mob/living/basic/farm_animals/pony.dm

@@ -119,6 +119,10 @@
 
 	whinny_angrily()
 
+/mob/living/basic/pony/get_shove_flags(mob/living/shover, obj/item/weapon)
+	. = ..()
+	. |= SHOVE_CAN_STAGGER
+
 /datum/ai_controller/basic_controller/pony
 	blackboard = list(
 		BB_TARGETING_STRATEGY = /datum/targeting_strategy/basic,