Handle unsupported /b en <type> entity mappings without stacktraces - #490

Fixes #458

/b en <type> could throw when the selected WorldEdit EntityType cannot be adapted to a Bukkit entity class on the running server version. This change converts that failure path into the existing user-facing “cannot spawn” flow.

• Entity adaptation hardening (EntityBrush#spawn)

  • Wrap BukkitAdapter.adapt(entityType).getEntityClass() with targeted exception handling (IllegalArgumentException).
  • On adaptation failure, emit voxelsniper.brush.entity.cannot-spawn and return early (no thrown stacktrace).

• Null entity class handling

  • If adaptation resolves but returns null entity class, emit the same cannot-spawn caption and exit before spawn attempts.

• Behavioral outcome

  • Unsupported or unmappable entity types now fail gracefully and consistently with existing spawn-failure messaging.

Class<? extends org.bukkit.entity.Entity> entityClass;
try {
    entityClass = BukkitAdapter.adapt(entityType).getEntityClass();
} catch (IllegalArgumentException e) {
    messenger.sendMessage(Caption.of("voxelsniper.brush.entity.cannot-spawn"));
    return;
}
if (entityClass == null) {
    messenger.sendMessage(Caption.of("voxelsniper.brush.entity.cannot-spawn"));
    return;
}