Mock out optional service libraries from test suite

[ryneeverett]

I don't believe they're exercised at all during tests so it should be a simple replacement with a generic stub. We can then update the developer docs to only require installing [test] dependencies rather than [all].

We should add a CI job to install all optional dependencies to make sure they continue to be resolvable once we're no longer doing so in development.

[Lotram]

On a related subject: dev dependencies could be moved to a "dependency-group" instead of an optional dependency, as defined in this PEP.
uv fully supports this