Upstream compatibility tests

[maxdebayser]

Since we test against "main" and "default" vLLM versions in our tests, we often add code to make "main" work ahead of our official support. But it's easy to forget this compatibility code and it can become technical debt later. In this PR I'm adding a new test file to that verifies "main" and "default" imports from vLLM so that the "default" tests starts failing one a compatibility code is not longer needed.

[github-actions]

👋 Hi! Thank you for contributing to vLLM support on Spyre.
Just a reminder: Make sure that your code passes all the linting checks, otherwise your PR won't be able to be merged. To do so, first install the linting requirements, then run format.sh and commit the changes. This can be done with uv directly:

uv sync --frozen --group lint --active --inexact

Or this can be done with pip:

uv pip compile --group lint > requirements-lint.txt
pip install -r requirements-lint.txt
bash format.sh

Now you are good to go 🚀

[joerunde]

LPGTM, would ideally like a smaller set of tests for backwards compatibility but we can get to that later

edit: Max is already on it 😎 ⚡⚡⚡