Test across different dependency versions

Libraries don't (or shouldn't) control the exact version of the dependencies they run with. Instead this is determined by the application's (charm's) lockfile at packing time. We should adjust the testing strategy and treatment of lockfiles in this repository to account for this. For example, we could use the different [uv lock --resolution](https://docs.astral.sh/uv/reference/cli/#uv-run--resolution) options when running unit tests.

Start by reviewing the repository's unit-test commands and how dependency lockfiles are used during testing. Investigate uv lock --resolution and determine how tests can run against different dependency resolutions without treating the library's lockfile as authoritative. Done means the testing strategy covers the supported dependency versions and lockfile handling matches application packing.

Written by the indexing model from the issue text.