CSS-9575 Add device flow integration tests

[kian99]

Description

This PR adds integration tests for login via the device flow testing that the session token login provider works as expected and extends our model proxy test suite. The JIMM test suite now has a channel that simulates the polling JIMM does when it queries for an access token from the OIDC server.

In order to do this I've extended the mock OAuthAuthenticator to implement the device flow methods.

Also added:

• A test for the model proxy that makes a juju status call.
• A test for the model proxy that tests what happens when a user tries to access a model they don't have permission to access.

Partially addresses CSS-9575

Engineering checklist

Check only items that apply

• Documentation updated
• Covered by unit tests
• Covered by integration tests