SET's mechanic doesn't register offhand items as part of set

DezForse
2025-12-04T22:42:56Z

Server Version
Paper version 1.21.10-113-main@9fc21bc (2025-11-14T16:11:13Z)

Plugin Version(s)
ProtocolLib v5.4.1-SNAPSHOT-5773bcb
MythicMobs v5.11.0-SNAPSHOT-4b0382d7
ModelEngine vR4.0.9

Issue description:
If you add set item for offhand (f.e. shield), take it to offhand - it wont be registered as part of set so effects from set won't work.

Steps to reproduce:
1. Use this amazing set at your server:
```
BABY_BOOBOO:
Enabled: true
Display: 'Baby Booboo Set'
Lore:
- '<#FF9999>Baby Booboo Set<&co>'
- '<#FF9999>Makes you small and fast'
Bonuses:
- Pieces: 2
Stats:
- SCALE 0.5 SETTER
- MOVEMENT_SPEED 1.2 MULTIPLY
```

2. Add two items for test:
```
test_iron_sword:
Id: iron_sword
EquipmentSet: BABY_BOOBOO
```
```
test_iron_shield:
Id: SHIELD
Material: SHIELD
EquipmentSet: BABY_BOOBOO
```

3. Take items in main and offhand and see nothing. Or even you may take two swords in both hands, which also should work cause we don't have any conditions for sets equipslot.

![image](/uploads/dcc48fe4bafae36c5dae7e489163036f/image.png)

Feel free to ask more details in DS: @DezForse

Thank you!