Motivation
As a pre-requisite to #241. Will also make tests a lot faster if we can avoid subprocess calls. Similar to how we use frozen=True by default, I think we could use use_uv=False by default for tests and just make exceptions for the cases where we need to have frozen=False - i.e. cases where we need to access the venv and actually install something. Hopefully this would speed up the test suite.

Summary of desired enhancement
Add internal config state which allows us to avoid a uv subprocess call. Will need to create alternative functions for each call; for example, instead of uv add we will need to manually add to the dependencies lists (without lower bounds, presumably).

This wouldn't be exposed in the interface anywhere, it would just be internal for now.

We wouldn't re-configure the tests to use this new config yet, that can be done later.