chore/Pull in unreleased pydantic Union bugfix (#65)

Why
===

An upstream bugfix in
pydantic/pydantic-core#1332 isn't able to be
installed due to tight version pinning in
[`[email protected]/pyproject.toml#L53`](https://github.com/pydantic/pydantic/blob/v2.8.2/pyproject.toml#L53).

For the time being, just switch to the latest release in the pydantic
repo as of the time of writing, which appears to resolve the generation
issue I'm seeing when running the codegen.


![image](https://github.com/user-attachments/assets/b85ea67c-91cc-455c-bcc1-176cce9610a8)

What changed
============

- Set pydantic version to track git @
pydantic/pydantic@f5d6acf

Test plan
=========

Manually running the codegen against production schemas appears to
generate sensible results

Author: blast-hardcheese