moves abstract_type up to datum, spawners wont spawn them (#92909)

## About The Pull Request
moves all implementations (im aware of) for "Im a parent type dont spawn
me please" to the datum layer to standardized behavior
adds a standerized proc for filtering out "bad" items that we dont want
spawning. applies to it the subtype vendor, gifts, and a new spawner and
mystery box for a random gun (neither playerfacing)
"port" of https://github.com/shiptest-ss13/Shiptest/pull/4621



https://github.com/user-attachments/assets/22f6f0b2-b44e-411a-b3dc-6b97dc0287aa

small warning: I dont have EVERY abstract type defined right now but,
ive done a good enough job for now. Im tired of data entry rn
## Why It's Good For The Game
standardizing behavior. Might be a micro hit to performance however

having this lets us not rely on icon state to determine whether
something is a parent type and makes it much easier to tell something is
a parent type (could be applied further to things like admin spawning
menus and things like that).

need feedback on if this is actually good for the game.
## Changelog
🆑
add: Soda cans show up in the silver slime drink table.
add: Examine tag for items that are not mean to show up ingame.
refactor: Standardizes how gifts rule out abstract types.
fix: gifts no longer check if something has an inhand, massively
expanding the list of potential items.
/🆑

code/modules/unit_tests/unit_test.dm

@@ -31,6 +31,9 @@ GLOBAL_VAR_INIT(focused_tests, focused_tests())
 	return focused_tests.len > 0 ? focused_tests : null
 
 /datum/unit_test
+	/// Do not instantiate if type matches this
+	abstract_type = /datum/unit_test
+
 	//Bit of metadata for the future maybe
 	var/list/procs_tested
 
@@ -47,9 +50,6 @@ GLOBAL_VAR_INIT(focused_tests, focused_tests())
 	var/list/allocated
 	var/list/fail_reasons
 
-	/// Do not instantiate if type matches this
-	var/abstract_type = /datum/unit_test
-
 	/// List of atoms that we don't want to ever initialize in an agnostic context, like for Create and Destroy. Stored on the base datum for usability in other relevant tests that need this data.
 	var/static/list/uncreatables = null