Discovery advertised the device_code grant in grant_types_supported, but dynamic
client registration only allowed authorization_code/refresh_token, so a client
listing the device grant in its RFC 7591 metadata was rejected with
invalid_client_metadata.
Introduce a single source of truth — OidcController::SUPPORTED_GRANT_TYPES —
used by discovery (grant_types_supported) and by registration validation, so
advertisement and registration can never drift. The token dispatcher already
handles exactly these grants for all clients (they are user-context grants gated
by consent + user_allowed?, so there is no per-client grant restriction), making
advertisement, registration, and enforcement consistent.
Tests: a client registering with the device_code grant now succeeds, plus an
assertion that registration's accepted set equals discovery's advertised set.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01F7cwhwDJp3MJJDoNPVE6zq
