Mob Spawn Control

Changes:

• SpawnController.removeSpawnGroup() added to all three targets — discards the mount (getVehicle()), all passengers, and the mob itself.
• ServerWorldMixin (1.21) / ServerLevelMixin (26.1/26.2) now evaluate the whole connected group on every spawn: each passenger and the vehicle is checked against the rules using its own captured spawn reason (fallback to the incoming mob's reason). If any member is banned, the entire group is discarded and the spawn returns false; replacement still runs only for the self-denied mob.
• Now banning minecraft:zombie kills both paths: the chicken (mount) spawn is denied because its zombie passenger is banned, and if the zombie spawn is evaluated first, the vehicle check catches the chicken too — no surviving jockey either way.