Add regression test confirming island home location is set on the correct island during concurrent creation

[Copilot]

When a player spams /is create, a race condition caused island 1's home location to be overwritten with island 2's spawn point. The bug: postCreationTask called setHomeLocation(user, spawn), which resolves the target island by UUID primary-island lookup — but by the time island 1's paste callback fires, island 2 creation has already updated the primary pointer, so the home lands on the wrong island.

The fix (setHomeLocation(island, spawn, "")) was already applied in the codebase, targeting the specific island object directly.

Changes

• Regression test (testBuilderWithSpawnPointSetsHomeOnIslandDirectly): makes the Bukkit scheduler execute the post-creation runnable inline, stubs a non-null spawn point, then asserts:
  • setHomeLocation(island, spawnLoc, "") is called with the exact island reference
  • setHomeLocation(user, ...) is never called during island creation

// BAD (original) — resolves via primary-island UUID lookup, wrong island under concurrency
plugin.getIslands().setHomeLocation(user, island.getSpawnPoint(Environment.NORMAL));

// GOOD (current) — targets the specific island object, immune to primary-island pointer races
plugin.getIslands().setHomeLocation(island, island.getSpawnPoint(Environment.NORMAL), "");