Fix: Shelf block entity items rendered in wrong slot for Bedrock clients

[LPettay]

Bedrock determines shelf item position by list index, not by a Slot tag. The previous implementation streamed Java's sparse Items list (which only contains occupied slots), so items would shift left into the wrong visual position. For example, an item in slot 1 (middle) would appear in slot 0 (left) on Bedrock.

This fix produces a dense 3-element list with empty item entries for unoccupied slots, so each item lands at the correct list index matching its Java Slot value.

Before: [{item}] → Bedrock renders in slot 0 (wrong)
After: [{empty}, {item}, {empty}] → Bedrock renders in slot 1 (correct)

Tested on a Velocity + Paper server with both Java and Bedrock clients connected.

[LPettay]

Video demonstration of issue pre/post fix

shelf-fix-demo.mp4

[onebeastchris]

Hello! Thank you for the PR. In the future, please indicate to which extent AI tools have been used - we'll get out AI policy online asap.

Tested BDS handling, which matches the changes implemented in this PR:

[16:32:30:462] [CLIENT BOUND] - BlockEntityDataPacket(blockPosition=(96, 63, 101), data={
  "Items": [
    {
      "Count": 64b,
      "Damage": 0s,
      "Name": "minecraft:shulker_shell",
      "WasPickedUp": 0b
    },
    {
      "Count": 0b,
      "Damage": 0s,
      "Name": "",
      "WasPickedUp": 0b
    },
    {
      "Count": 64b,
      "Damage": 0s,
      "Name": "minecraft:nautilus_shell",
      "WasPickedUp": 0b
    }
  ],
  "id": "Shelf",
  "isMovable": 1b,
  "x": 96i,
  "y": 63i,
  "z": 101i
})

[onebeastchris]

LGTM