Integration tests should be migrated to `jubilant`

### Context

Current integration tests are using `pytest-operator` to interact with Juju. It is recommended to migrate to [Jubilant](https://documentation.ubuntu.com/jubilant/).

### What needs to get done

1. Migrate integration tests to `jubilant`

### Definition of Done

1. Integration tests are passing

Start by reviewing the repository's integration-test suite and the Jubilant documentation linked in the issue, comparing its interfaces with the current pytest-operator usage. Migrate the integration tests to Jubilant, then run the integration-test suite; done means all integration tests pass.

Written by the indexing model from the issue text.