sign_in and link now take only identityProviders; connect takes only
managedConnectionProviders. supportsProvider(), which unioned the role lists
with entitlementSources and never looked at the mode, is deleted: a Joystick
sign_in for streampixels no longer starts, so a Deru sign-in cannot rewrite
his managed credential row. No built-in profile had a provider reachable
only through the union.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>