CI fix: Fix CI failure due to incorrect attribute access in models.py

This commit addresses a CI failure caused by an incorrect attribute access in `spice/models.py`. The error was due to attempting to access the `model_copy` attribute on a class that does not have it. The fix involves reverting the change to use `dataclasses.replace` for the `Model` class, which is still a dataclass, while keeping the `model_copy` usage for the `SpiceCallArgs` class, which has been converted to a Pydantic model. This ensures compatibility and correct behavior for both dataclasses and Pydantic models in our codebase.