Use pytest-jubilant in integration tests and upload juju debug-log

With the release of `pytest-jubilant` 2.0, we have a nice way to capture the `juju debug-log` on model teardown. We should use this in `charmlibs`.
- Require `pytest-jubilant` based integration tests (document, update existing tests).
- Update the template and example libraries to use `pytest-jubilant`.
- Pass `--juju-dump-logs` in CI and use `actions/upload-artifact` to export the logs nicely.

Start by locating the integration tests, their dependency documentation, the template and example libraries, and the CI workflow. Check how pytest-jubilant is currently configured, then verify that the tests require it, CI passes --juju-dump-logs, and uploaded artifacts contain the Juju debug logs.

Written by the indexing model from the issue text.