refactor `model` parameter type to be enum

We currently have error handling for when the `model` string input doesn't match our dictionary, but can eventually refactor to make it an enum that matches one of the below model names in our constants dictionary to make it even more explicit:

https://github.com/AgentOps-AI/tokencost/blob/main/tokencost/constants.py

Not too familiar with Python so I'm not sure if code editors have shortcuts for viewing definitions of enums like TypeScript does, but our code is easy enough to read that in our error messages as well as the code itself we just redirect people to the constants file