Infinite villager trading in multiplayer using boats

The bug

When a player leaves the server while in a boat that has a villager in it, other players that have previously accessed the villager's UI will still be able to interact with it, even after the villager disappears along with the boat and the player leaving the server.

The same bug also applies to donkeys and mules, see MC-161754.

To reproduce

1. Put a villager inside a boat
2. Player 1 enters the boat
3. Player 2 interacts with the villager
4. Player 1 leaves the server
   → Player 2's villager UI stays open
5. Player 2 trades with the villager
   → Player 2 is able to successfully complete the trades
6. Player 1 joins the server
   → The villager appears again, but the trades Player 2 did did not register