feat: Add integration tests

[alithethird]

Add integration tests for health check and oauth.

This PR also adds capability to run those integration tests and linting in the CI and in local.

Local validation and integration test guide

Run formatting

From repo root:
tox -e fmt

Run lint checks

From repo root:
tox -e lint

Run unit tests

From repo root:
tox -e unit
Note: in the current tox.ini, the unit env is a placeholder command, so this mainly validates the env wiring.

Build artifacts required for integration tests

Build rock:

```
cd dashboard
rockcraft pack
```

Build charm:

```
cd charm
charmcraft pack
```

Push image to local registry for Juju deployment

Example:

cd dashboard
rockcraft.skopeo --insecure-policy copy --dest-tls-verify=false oci-archive:dashboard_0.51_amd64.rock docker://localhost:32000/dashboard:0.51

Run integration tests

From repo root:
tox -e integration -- --charm-file=charm/dashboard_ubuntu-22.04-amd64.charm --dashboard-image=localhost:32000/dashboard:0.51

Optional: run specific integration suites

Health checks only:

tox -e integration -- test_charm.py --charm-file=charm/dashboard_ubuntu-22.04-amd64.charm --dashboard-image=localhost:32000/dashboard:0.51

OAuth flow only:

tox -e integration -- test_oauth.py --charm-file=charm/dashboard_ubuntu-22.04-amd64.charm --dashboard-image=localhost:32000/dashboard:0.51

Model behavior notes

Integration fixtures in conftest.py support temporary model creation by default.
You can also pass --use-existing or --model if you want to reuse a model.
OAuth flow in test_oauth.py triggers Playwright browser setup via fixtures.

---

Manual checks

• If you changed the Dashboard application or the rock, have you increased the version number in rockcraft.yaml? Remember to use the same version number in README.md.