Update dataclasses.replace calls in DefaultRetryStrategy

This commit further addresses issue #111 by updating additional `dataclasses.replace` calls to `call_args.model_copy(update={...})` in the `spice/models.py` file. This change is necessary because `SpiceCallArgs` has been converted from a dataclass to a Pydantic model. The updates ensure that the model copying mechanism is consistent across the codebase. 

Additionally, the unnecessary import of `dataclasses` has been removed from `default_strategy.py`.

spice/models.py

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

spice/retry_strategy/default_strategy.py

@@ -1,4 +1,3 @@
-import dataclasses
 from typing import Any, Callable, Optional
 
 from spice.call_args import SpiceCallArgs