[MIRROR] Fix a few vendor restocking bugs

[Steals-The-PRs]

Mirrored on Nova: NovaSector/NovaSector#2008
Original PR: tgstation/tgstation#82603

About The Pull Request

So while working on fixing standard RPEDs working at all, I noticed they weren't displaying parts when used on a vendor.
Looking into it, it seemed to be because the vendors just called display_parts(user) on its own:
https://github.com/tgstation/tgstation/blob/1780fcef62e0413da65375a37f7c8030a0371419/code/modules/vending/_vending.dm#L1151
While other machinery would wrap it in a to_chat(...) call.
https://github.com/tgstation/tgstation/blob/1780fcef62e0413da65375a37f7c8030a0371419/code/game/machinery/_machinery.dm#L971
So, we do that too!

But then, during further testing, I noticed it wasn't actually restocking. Ever. Even though it's calling the restock(...) proc with the same inputs as you would do manually- oh. oh.
https://github.com/tgstation/tgstation/blob/1780fcef62e0413da65375a37f7c8030a0371419/code/modules/vending/_vending.dm#L1152-L1154
It's checking the replacer for whether it's a refill canister, and not the contents of it. Has been doing this for about 8 months, apparently. No wonder it wasn't working.

Great! This should work, right? One more round of testing!
Aaaaand it doesn't actually pay out any credits when restocking this way.
That's because it's specifically coded on attackby(...).
https://github.com/tgstation/tgstation/blob/1780fcef62e0413da65375a37f7c8030a0371419/code/modules/vending/_vending.dm#L707-L715
Alright, well, we move this to a new post_restock(...) proc, and call this whenever we're done doing restocks.

/obj/machinery/vending/proc/post_restock(mob/living/user, restocked)
	if(!restocked)
		to_chat(user, span_warning("There's nothing to restock!"))
		return

	to_chat(user, span_notice("You loaded [restocked] items in [src][credits_contained > 0 ? ", and are rewarded [credits_contained] credits." : "."]"))
	var/datum/bank_account/cargo_account = SSeconomy.get_dep_account(ACCOUNT_CAR)
	cargo_account.adjust_money(round(credits_contained * 0.5), "Vending: Restock")
	var/obj/item/holochip/payday = new(src, credits_contained)
	try_put_in_hand(payday, user)

This is separate from the restock(...), so we can call it when we're done restocking and need a message, rather than sending one for each canister in our RPED. We can't just break the loop on the first fitting canister, because the items we need might be spread over multiple partially full canisters.

Third round of testing anyone? Oh hey infinite money glitch.
It seems like we never actually reset the credits_contained var after paying out with it.
Soooo we just append this to our post_restock(...) proc, and be done with it.

/obj/machinery/vending/proc/post_restock(mob/living/user, restocked)
	(...)
	try_put_in_hand(payday, user)
	credits_contained = 0

At this point, I felt it better we also reorganized the vendor exchange_parts(...) to only have one display_parts(...) call and an early return if we can't exchange parts.

Any further issues I could find I felt were outside of the scope, and better off atomized into a separate pr.

Why It's Good For The Game

Fixes RPEDs not working with vendors at all.
Fixes RPEDs not displaying vendor parts.
Fixes restocking vendors with an RPED not giving credits.
Fixes vendors not resetting their contained credits when restocked.

Changelog

🆑 00-Steven
fix: RPEDs can be used on vendors again. Note that only bluespace RPEDs can carry vendor refills as of writing.
fix: RPEDs can display vendor parts again.
fix: Restocking vendors gives credits whether done manually or by RPEDs.
fix: Vendors reset their contained credits when restocked.
/:cl: