Fix race condition in SoulboundListener Folia task scheduling

[Copilot]

The containsKey → runAtEntityLater → put sequence in onRespawn(EntityDeathEvent) was non-atomic, allowing two concurrent death events for the same player to each pass the guard check and schedule duplicate soulbound-return tasks.

Changes

• returnTasks: HashMap → ConcurrentHashMap; replaced containsKey + put with computeIfAbsent to atomically guarantee at most one scheduled task per UUID
• soulbound: HashMap → ConcurrentHashMap to cover concurrent access between the death handler and respawn handler on Folia

// Before — not atomic; two threads can both pass containsKey and schedule
if (returnTasks.containsKey(p.getUniqueId())) { return; }
WrappedTask task = Slimefun.getPlatformScheduler().runAtEntityLater(p, () -> { ... }, 10L);
returnTasks.put(event.getEntity().getUniqueId(), task);

// After — single atomic check-and-insert
returnTasks.computeIfAbsent(p.getUniqueId(), uuid ->
    Slimefun.getPlatformScheduler().runAtEntityLater(p, () -> { ... }, 10L));

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