Vellum-ai

Bump versions of Fern and Python Sdk Generator

Removes support for the legacy retrieve model version API

Remove support for the deprecated register-prompt endpoint.

1. Converts our Upsert Prompt Scenario api to use v2 format instead of v1.
2. Exposes the Retrieve Document API
3. Converts all of our prompt data model to be truly polymorhpic
4. Allows for function calling in test cases
5. Exposes the new "NULL" variable type.
6. Exposes `vellum.lib` in our python sdk - our new suite of helper classes for working with our API.

Fix pytest-httpx dependency

Adds pytest_httpx to our python client