fix asset loading subsystem (#92508)

## About The Pull Request
The entire asset loading had multiple issues.

Firstly, we could hit a tick check after an asset was generated and
didn't remove the index, leading to the asset being generated twice.

Secondly, we had the issue that the icon forge batching uses UNTIL to
wait until generation finishes. This led to the situation that the
entire subsystem fire proc was paused outside of the internal tick check
pause.
## Why It's Good For The Game
Especially on larger sprite sheets or when there're many to be generated
this led to the situation that we looked up a rust job multiple times:
 
`[2025-08-08T18:42:11] Runtime in
code/modules/asset_cache/iconforge/batched_spritesheet.dm,200:
Spritesheet design UNKNOWN ERROR: NO SUCH JOB`

## Changelog
No player facing changes.

code/modules/asset_cache/spritesheet/batched/batched_spritesheet.dm

@@ -232,7 +232,7 @@
 		CRASH("Error during spritesheet generation for [name]: [data["error"]]")
 
 /datum/asset/spritesheet_batched/queued_generation()
-	realize_spritesheets(yield = TRUE)
+	INVOKE_ASYNC(src, PROC_REF(realize_spritesheets), TRUE) // The proc is called inside a subsystem and waits with an UNTIL
 
 /datum/asset/spritesheet_batched/ensure_ready()
 	if(!fully_generated)