Fixes #11921 | Consider off-hand when adding items to player inventory

[niho2]

Problem

The addItem method in CraftInventory doesn't consider the off-hand when adding items to a full player inventory. When an item is partially in the off-hand and the rest of the inventory is full, it incorrectly indicates that there's no space available, even though there might be space in the off-hand.

Changes

• Extended the addItem method to check if it's a player inventory
• Added additional check for the off-hand when no partial stack is found in the main inventory
• Attempts to stack items in the off-hand before looking for an empty slot
• Maintains original logic for non-player inventories

Test Cases

1. Player has an item in off-hand with free stack space
2. Player attempts to add the same item
3. Item is correctly added to the off-hand
4. Remaining items (if any) are placed normally in the inventory

Additional Notes

• Change respects the maximum stack size of the item
• No API surface changes
• Backwards compatible with existing functionality

[github-actions]

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