Add selection-based tenant resolver and tenant selection page flow with single-tenant auto-select

[Copilot]

Summary

Adds a selection-based tenant resolver with an overridable tenant selection page flow, and now auto-selects the tenant when the endpoint returns exactly one option.

Added

• New Selection tenant resolution strategy and typed SelectionOptions with configurable TenantsEndpoint
• New TenantSelectionMiddleware that:
  • fetches selectable tenants from the configured endpoint
  • writes .cratis-tenants for page rendering
  • serves select-tenant.html when no tenant is selected
  • handles /.cratis/select-tenant?tenantId=...&returnUrl=... and writes .cratis-tenant
  • auto-selects and persists tenant when exactly one tenant is returned, skipping selector UI
• New built-in overridable page: select-tenant.html
• New docs page: Documentation/configuration/tenant-selection.md
• New specs for selection strategy and tenant-selection middleware behavior, including single-tenant auto-select

Changed

• Extended well-known constants:
  • cookie names (.cratis-tenants, .cratis-tenant)
  • page name (select-tenant.html)
  • path (/.cratis/select-tenant)
• Registered selection strategy in tenancy DI and options binding
• Updated tenant resolver to support Selection as direct tenant-id resolution
• Updated configuration docs/toc to include tenant selection and page override guidance
• Updated tenant-selection docs flow to describe single-tenant auto-selection behavior

Fixed

• Prevented open-redirect risk in tenant selection callback by enforcing local relative return URLs before redirecting

Removed

• None

Security

• Hardened tenant selection redirect path handling to reject non-local return targets

Deprecated

• None

{
  "Cratis": {
    "AuthProxy": {
      "TenantResolutions": [
        {
          "Strategy": "Selection",
          "Options": {
            "TenantsEndpoint": "https://platform.example.com/api/tenants/selectable"
          }
        }
      ]
    }
  }
}

[einari]

@copilot if there is only one tenant in the list, use that and don’t show selector

[Copilot]

@copilot if there is only one tenant in the list, use that and don’t show selector

Implemented in f5e73a3. When tenant selection returns exactly one tenant, AuthProxy now sets .cratis-tenant and redirects back to the original URL instead of showing select-tenant.html. Added spec coverage for this behavior.