Make player and island metadata maps thread-safe

[tastybento]

Problem

A server admin reported console spam on every PlayerMoveEvent with BentoBox 3.22.3 and Border 4.8.5 (https://mclo.gs/h623Vin):

java.util.NoSuchElementException
	at java.util.HashMap$HashIterator.nextNode(HashMap.java:1608)
	at java.util.HashMap$KeyIterator.next(HashMap.java:1629)
	at world.bentobox.bentobox.database.objects.Players.isImmutable(Players.java:272)
	at world.bentobox.bentobox.database.objects.Players.getMetaData(Players.java:259)
	at world.bentobox.bentobox.api.user.User.getMetaData(User.java:1226)
	at world.bentobox.bentobox.api.metadata.MetaDataAble.getMetaData(MetaDataAble.java:31)
	at world.bentobox.border.listeners.PlayerListener.isOn(PlayerListener.java:229)

Players.getMetaData() called isImmutable(map) on every read, and that helper wrote to the map each time (it re-put the first existing key to see whether an UnsupportedOperationException was thrown). The map was a plain HashMap with no synchronization, so every metadata read anywhere in the ecosystem was a concurrent-unsafe write. If any other thread touched the map at the same time, the HashMap could end up with size > 0 but an empty bucket table. From then on !map.isEmpty() was true but keySet().iterator().next() found nothing and threw, on every read, until restart.

Fix

• Players and Island copy metadata into a ConcurrentHashMap once, on first access or on setMetaData, and never mutate on read. The instanceof ConcurrentMap check replaces the write-probe, and also covers immutable maps coming from deserialization.
• setMetaData copies the caller's map instead of adopting it, so immutable maps are accepted (previously Players.setMetaData threw IllegalArgumentException) and the caller's map is never mutated.
• The Island copy constructor uses the same helper.
• MetaDataAble.putMetaData(key, null) now removes the key, because a concurrent map cannot hold null values. Null keys/values are dropped when copying.

Tests

• New PlayersTest covering lazy init, immutable-map copy, caller-map isolation, null handling, and a multi-threaded hammer test that exercised the old corruption path.
• IslandTest gains equivalent immutable-map, caller-isolation and copy-constructor tests.
• Full suite: 3518 tests, 0 failures.

🤖 Generated with Claude Code