Migrate the PowerBI dashboard connector's test-connection off the legacy test_connection_steps helper onto the declarative @check / ChecksProvider / ErrorPack framework.

PowerBI is the FIRST Dashboard-vertical connector to migrate, so this also seeds the shared Dashboard scaffolding (checks/dashboard.py: DashboardStep enum + REST-generic helpers) that Tableau and Looker will reuse.

Scope:

• Add DashboardStep (CheckAccess, GetDashboards) + reusable REST helpers.
• Replace the test_connection override on PowerBIConnection with checks().
• Add a CheckAccess OAuth gate step (ConnectionGate) to powerbi.json.
• HTTP-status-aware error pack folding in the shared network pack.