Refactored the test

[minulo]

What this PR does

Refactored the get-proxied-endpoints Juju action tests so that it fits

Why we need it

To ensure all the tests in the code base follow the same style

Checklist

• I followed the contributing guide
• I added or updated the documentation (if applicable)
• I updated docs/changelog.md with user-relevant changes
• I added a change artifact for user-relevant changes in docs/release-notes/artifacts. If no change artifact is necessary, I tagged the PR with the label no-release-note.
• I used AI to assist with preparing this PR
• I added or updated tests as needed (unit and integration)
• If integration test modules are used: I updated the workflow configuration
  (e.g., in .github/workflows/integration_tests.yaml, ensure the modules list is correct)
• If this PR involves a Grafana dashboard: I added a screenshot of the dashboard
• If this PR involves Terraform: terraform fmt passes and tflint reports no errors

[Copilot]

@copilot I just did git pull. Can you cjeck on the get_proxied_endpoints_with_backend_filter_non_existing_backend before any our changes vs after? Our aseert conmments aint adding up with what is curr...

Before and after the PR, this test asserted {"endpoints": "[]"}; the refactor did not change that behavior or assertion. The action implementation filters unmatched backends and always sets JSON results, so it returns an empty list rather than raising ActionFailed. The stale part is the docstring’s assert: raises ActionFailed text; the assertion and actual code are consistent.