Fix plain SHULKER_BOX not treated as shulker box, voiding its contents

[Copilot]

Non-dyed shulker boxes (SHULKER_BOX) were being sold as a regular item for their flat price while their contents were silently destroyed. Only dyed variants (WHITE_SHULKER_BOX, etc.) were correctly recognized and had their contents sold.

Root cause

isShulkerBox matched only on endsWith("_SHULKER_BOX"), which excludes the uncolored SHULKER_BOX material name. It fell through to the normal item-sell path, consumed the slot, and voided everything inside.

Fix

// Before
return item != null && item.getType().name().endsWith("_SHULKER_BOX");

// After
String name = item.getType().name();
return name.equals("SHULKER_BOX") || name.endsWith("_SHULKER_BOX");