Correctly handle ender pearl end gateway teleportations

The end gateway is supposed to teleport the person who threw
the ender pearl.

The changes more closely mirror Vanilla behavior. The current
exceptions to Vanilla behavior are:

1. The first teleportation attempt for the end gateway always fails
2. If the ender pearl thrower is riding a vehicle, the thrower is
   dismounted from their vehicle.

I don't see any solutions for #1 right now. The root issue is that
since the end gateway does not have a target location, it has to
search for one. However, it can _fail_ to find a target location,
in which case the teleportation should not occur. Since the search
must take place asynchronously, it requires the entity to be
removed from the world.

For #2, this is because Vanilla's behavior is broken and does not
correctly teleport players riding boats. We can fix this by simply
dismounting the player and teleporting them separately of their boat,
which seems to be what Vanilla is trying to do given it does _not_
try to teleport the root vehicle of the player.

This is a partial fix to PaperMC#51

patches/server/0003-Threaded-Regions.patch

@@ -20830,7 +20830,7 @@ index 8b2a02f85085c91c51b61760de967a859bf5e4de..eb8099e463eaae6268c4cb349ba88c11
 
      @Override
 diff --git a/src/main/java/net/minecraft/world/level/block/entity/TheEndGatewayBlockEntity.java b/src/main/java/net/minecraft/world/level/block/entity/TheEndGatewayBlockEntity.java
-index d9baa85962236c42219cf09d4f3129be93ff069c..cc01030a2aa6aade37ed7d9e45004e33ae9a0f81 100644
+index d9baa85962236c42219cf09d4f3129be93ff069c..f23174be9d198c9aaf654ff39483ecec5104a9e1 100644
 --- a/src/main/java/net/minecraft/world/level/block/entity/TheEndGatewayBlockEntity.java
 +++ b/src/main/java/net/minecraft/world/level/block/entity/TheEndGatewayBlockEntity.java
 @@ -51,9 +51,12 @@ public class TheEndGatewayBlockEntity extends TheEndPortalBlockEntity {
@@ -20856,7 +20856,7 @@ index d9baa85962236c42219cf09d4f3129be93ff069c..cc01030a2aa6aade37ed7d9e45004e33
      }
 
      public boolean isSpawning() {
-@@ -176,8 +179,112 @@ public class TheEndGatewayBlockEntity extends TheEndPortalBlockEntity {
+@@ -176,8 +179,136 @@ public class TheEndGatewayBlockEntity extends TheEndPortalBlockEntity {
          }
      }
 
@@ -20921,12 +20921,36 @@ index d9baa85962236c42219cf09d4f3129be93ff069c..cc01030a2aa6aade37ed7d9e45004e33
 +            return;
 +        }
 +
++        Entity chosenEntity;
++        if (entity instanceof ThrownEnderpearl pearl) {
++            Entity owner = pearl.getOwner();
++
++            if (owner instanceof ServerPlayer player) {
++                CriteriaTriggers.ENTER_BLOCK.trigger(player, state);
++            }
++
++            if (owner != null) {
++                // vanilla behavior is to just break if the owner is riding anything
++                // it's not likely intentional that throwing a pearl while riding something is intended
++                // to teleport the vehicle, rather just the owner given the lack of getRootVehicle
++                owner.unRide();
++                chosenEntity = owner;
++                pearl.discard();
++            } else {
++                // see above for unRide()
++                pearl.unRide();
++                chosenEntity = pearl;
++            }
++        } else {
++            chosenEntity = entity.getRootVehicle();
++        }
++
 +        // This needs to be first, as we are only guaranteed to be on the corresponding region tick thread here
 +        TheEndGatewayBlockEntity.triggerCooldown(world, pos, state, blockEntity);
 +
 +        if (isExactTeleport) {
 +            // blind teleport
-+            entity.teleportAsync(
++            chosenEntity.teleportAsync(
 +                serverWorld, Vec3.atCenterOf(teleportPos), null, null, null,
 +                PlayerTeleportEvent.TeleportCause.END_GATEWAY, Entity.TELEPORT_FLAG_LOAD_CHUNK | Entity.TELEPORT_FLAG_TELEPORT_PASSENGERS,
 +                (Entity teleportedEntity) -> {
@@ -20940,7 +20964,7 @@ index d9baa85962236c42219cf09d4f3129be93ff069c..cc01030a2aa6aade37ed7d9e45004e33
 +            // should be teleported, something something else...
 +            // however, we know the target location cannot differ by one region section: so we can
 +            // just teleport and adjust the position after
-+            entity.teleportAsync(
++            chosenEntity.teleportAsync(
 +                serverWorld, Vec3.atCenterOf(teleportPos), null, null, null,
 +                PlayerTeleportEvent.TeleportCause.END_GATEWAY, Entity.TELEPORT_FLAG_LOAD_CHUNK | Entity.TELEPORT_FLAG_TELEPORT_PASSENGERS,
 +                (Entity teleportedEntity) -> {
@@ -20962,14 +20986,14 @@ index d9baa85962236c42219cf09d4f3129be93ff069c..cc01030a2aa6aade37ed7d9e45004e33
          if (world instanceof ServerLevel && !blockEntity.isCoolingDown()) {
 +            // Folia start - region threading
 +            if (true) {
-+                teleportRegionThreading(world, pos, state, entity.getRootVehicle(), blockEntity);
++                teleportRegionThreading(world, pos, state, entity, blockEntity);
 +                return;
 +            }
 +            // Folia end - region threading
              ServerLevel worldserver = (ServerLevel) world;
 
              blockEntity.teleportCooldown = 100;
-@@ -281,6 +388,129 @@ public class TheEndGatewayBlockEntity extends TheEndPortalBlockEntity {
+@@ -281,6 +412,129 @@ public class TheEndGatewayBlockEntity extends TheEndPortalBlockEntity {
          return TheEndGatewayBlockEntity.findTallestBlock(world, blockposition1, 16, true);
      }