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 `models.py`. The error was due to attempting to access the `model_copy` attribute on a dataclass, which does not exist. The fix involves reverting the change to use `dataclasses.replace` for the `Model` class while keeping the `model_copy` method for the `SpiceCallArgs` class, which is a Pydantic model. This ensures compatibility and correct behavior for both dataclasses and Pydantic models in our codebase.