Add tox environments with unpinned dependencies for local testing

I want to use `UV_OFFLINE=1` to make things snappier, and the restrictions need to be less strict for offline mode to work.

Start by locating the repository's tox configuration and the dependency restrictions relevant to UV_OFFLINE=1. Add local-testing environments with unpinned dependencies, then verify that they can run successfully in offline mode.

索引モデルが issue の本文から書いたものです。