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Use pydantic UUID4 type to validate UUID strings
This works fine for prasing strings:
```
In [9]: UserConcreteObjectStoreModel(uuid=uuid.uuid4().hex, private=False, quota={"enabled": False}, badges=[])
Out[9]: UserConcreteObjectStoreModel(object_store_id=None, private=False, name=None, description=None, quota=QuotaModel(source=None, enabled=False), badges=[], device=None, uuid=UUID('c7a6751b-4d78-4d7f-b3b1-b2baf0a50d90'))
```
Fixes https://github.com/galaxyproject/galaxy/issues/18684#event-13846279733