Fix race conditions, NPEs and command logic in ItemManager

[google-labs-jules]

Refactored ItemManagerCommand to be stateless by removing instance fields and passing sender/args as parameters. Added argument length checks and type checks (instanceof Player) to prevent exceptions. Renamed 'give' command to 'get' to match help text and intended behavior. Added null check in ItemSort.getItem to prevent NPE.

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PR created automatically by Jules for task 10680733511993001918 started by @acsoto

[google-labs-jules]

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