[CI] Update GitHub Actions workflow to test service key authentication

[ThePlenkov]

Update .github/workflows/ci.yml to add an integration step that:

• Reads ADT_SERVICE_KEY and/or COPILOT_ADT_SERVICE_KEY secret (already present in repo)
• Runs adt auth login non-interactively in CI using the env var to ensure service key auth works
• Fails CI if service key auth is broken
• Exercise both (basic + service key) auth flows in CI

Parent Epic: #70
Blocks: #cli-auth-selection