Fix: reopen sell-confirm GUI on player region thread (Folia/Canvas crash)

[Potenza7]

Problem

On Folia/Canvas, confirming a spawner sell crashes when the previous GUI is reopened:

Thread failed main thread check: Cannot init menu async
  at net.minecraft.server.level.ServerPlayer.initMenu
  at ...SpawnerSellConfirmListener.reopenPreviousGui(SpawnerSellConfirmListener.java:197)
  at ...SpawnerSellConfirmListener.lambda$handleConfirm$1(SpawnerSellConfirmListener.java:174)
  at ...Scheduler.lambda$runTask$0 ... FoliaGlobalRegionScheduler

Cause

In handleConfirm(), the post-sell onComplete reopens the GUI via Scheduler.runTask (the global region scheduler). But player.openInventory() → ServerPlayer.initMenu() must run on the player's own region thread, so Folia/Canvas fails the TickThread check.

Fix

Dispatch the reopen via Scheduler.runEntityTask(player, ...) instead, so it runs on the player's region thread.

• Paper: falls back to the main thread (unchanged behavior).
• If the player logged off mid-sell, the entity task simply never runs (safe).

One-line change in SpawnerSellConfirmListener#handleConfirm (plus comment updates).

Tested on Canvas 26.1.2 — crash gone, GUI reopens correctly after selling.

[Dueris]

I can confirm(Canvas dev) as someone who's worked with Folia for a long time, all things that involve an entity instance should be scheduled to the entity in question. The global tick should never be used for that, as it's used for states that are global and doesn't have a specified location. Think time/weather for example.

If there are other places in the codebase improperly doing this, it'd be recommended to fix that, otherwise it can cause a lot of issues that are often, frankly, undiagnosable.