Investigate ephemeral and package-independent venvs for `mypy` and `ruff`

This is super low prio and more a fun python packaging/tooling side quest.

The aim is to avoid polluting the package-under-test's venv with otherwise unused develeper deps in order to cleanly verify the package and its dependencies are correct.

This is pretty close:
```
uv venv package_venv
source package_venv/bin/activate.fish
uv pip install -e packages/testing/
uv pip install -e .
uvx --with 'mypy==1.17.0' --with 'types-requests>=2.31,<2.33' --with pydantic mypy --config-file ./pyproject.toml --python-executable package_venv/bin/python3
```