fix(items): optimize inventory checks and fix free space calculation

[theEvilReaper]

Proposed changes

This PR refactors the Items utility class to improve performance and correctness when analyzing player inventories.

Previously, getAmountFromItem and getFreeSpace queried the inventory array repeatedly during loop iterations, creating unnecessary overhead. Additionally, getFreeSpace assumed all non-empty slots had a maximum stack size of 64, which led to incorrect free space calculations for items with custom stack limits such as Ender Pearls or unstackable tools and armor

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

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• I have read the CONTRIBUTING.md
• I have added tests that prove my fix is effective or that my feature works
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