Incorporate feedback to maintain dataclass usage for Model class

Reverted changes in `spice/models.py` to keep using `dataclasses.replace` for the `Model` class, as it remains a dataclass. Updated only the `SpiceCallArgs`-related code in `DefaultRetryStrategy` to use `model_copy` for Pydantic model updates. This ensures correct handling of both dataclasses and Pydantic models.

spice/models.py

@@ -174,9 +174,7 @@ def get_model_from_name(model_name: str) -> Model:
                 else:
                     input_cost = None
                     output_cost = None
-                model = model.model_copy(
-                    update={"name": model_name, "input_cost": input_cost, "output_cost": output_cost}
-                )
+                model = dataclasses.replace(model, name=model_name, input_cost=input_cost, output_cost=output_cost)
                 return model
 
     return UnknownModel(model_name, None)