Persistent trader llamas will still despawn when leashed to wandering traders

Per this comment by @unknown, trader llamas which are marked as persistent should not despawn when leashed to wandering traders. However, this is not the case. This issue prevents the player from being able to artificially create naturally spawned wandering trader with its two trader llamas.

Steps to Reproduce:

1. Summon a persistent, leashed trader llama:

   /summon trader_llama ~ ~ ~ {Leashed:1b,PersistenceRequired:1b}

2. Spawn a wandering trader

3. Leash the llama to the wandering trader:

   /data modify entity @n[type=minecraft:trader_llama] leash.UUID set from entity @n[type=minecraft:wandering_trader] UUID

Observed Behavior:

The trader llama will despawn when leashed, despite being marked as persistent.

Expected Result:

The trader llama would not despawn, but instead would be leashed to the wandering trader as expected.

Screenshots/Videos:

Notes:

Related to MC-258553