Ensure model_fields is called on a BaseModel subclass

[ocktowen]

📒 Description

In some cases get_fields receives an instance of a pydantic model. Accessing the model_fields attribute from the instance is deprecated and it should be accessed from the class.

🔗 What I've Done

I added an extra check to ensure the obj is always a class.

🛫 Checklist

• [ ] Updated docs
• [ ] Added unit-tests