Fix border teleportation bypass when player is outside all island spaces

[Copilot]

Players pushed by a piston past the barrier (e.g., falling below y=-64 then flying up) could move freely without being teleported back. Root cause: getIslandAt(to) returns Optional.empty() when the player is completely outside any island's space, which made both outsideCheck() and the backtrack logic silently no-op.

Changes

• outsideCheck() — return true when getIslandAt is empty (player not on any island), not just when they're on an island but outside its protection zone:

  Optional<Island> islandAt = addon.getIslands().getIslandAt(to);
  return islandAt.isEmpty() || islandAt.filter(i -> !i.onIsland(to)).isPresent();

• Backtrack teleport — fall back to getIsland(World, User) to resolve the player's own island when getIslandAt returns empty at their current location
• Test — added case covering a player completely outside any island space being teleported back via their own island lookup

Original prompt

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This section details on the original issue you should resolve

<issue_title>No teleportation</issue_title>
<issue_description>### Expected behavior

I think the barrier plugin should teleport the player back even if they are below -64 height.

Observed/Actual behavior

My players found a bug where if you push yourself over a barrier with a piston and fall below -64 height, and then fly up, you can safely fly behind the barrier without fear of being teleported back.

This raises problems because people can enter other islands and destroy them, look at other people's farms, etc.

Steps/models to reproduce

1. Push yourself over the barrier with the piston
2. Type /fly
3. Fly to other islands

BentoBox version

BentoBox-3.11.1
Border-4.6.0

Plugin list

AOneBlock-1.22.0
Biomes-2.2.1
Level-2.22.0
TopBlock-1.1.0
Warps-1.17.0

Other

I made a video for visual viewing.
https://youtu.be/EroVKnnx18g</issue_description>

<agent_instructions>Somehow the user is not being detected in their movement when outside the island. It's not clear how the <-64 location has an affect as the island check is done on x,z and not y as far as I can tell. Try and see if there is some way. Also, it's not clear how the teleportation back is not working because that should always work. There may be an edge case where the border is at the exact range of the island so when the user is pushed outside of the border, they are not in an island space at all.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes No teleportation #156

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[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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