[Client] Implement Dynamic Client Registration (RFC 7591) + pre-registered client support

Context

When no client credentials are pre-configured and AS metadata advertises registration_endpoint, the client must POST client_metadata to register itself dynamically. When credentials are pre-provided via Builder, skip registration.

Scope

• Mcp\Client\Auth\DynamicClientRegistrar:
  • POST client_metadata (client_name, redirect_uris, grant_types, token_endpoint_auth_method, software_id, software_version) to registration_endpoint.
  • Persist returned client_id / client_secret / client_secret_expires_at via TokenStorageInterface.
• Client\Builder::setOAuthClient(string $clientId, ?string $clientSecret = null) for pre-registration.

Conformance scenarios unblocked

auth/pre-registration (and unblocks every other auth scenario by providing a client_id).

Dependencies

Blocked by: #318 (AS metadata gives registration_endpoint).

Acceptance

• Unit tests for registration request shape + pre-registration bypass.
• Conformance: auth/pre-registration passes.

cc @soyuka

Comienza con el trabajo de metadatos de AS en #318 y, después, inspecciona Mcp\Client\Auth\DynamicClientRegistrar, Client\Builder y TokenStorageInterface. Ejecuta el escenario de conformidad de auth/pre-registration y añade las pruebas unitarias solicitadas para la estructura de la solicitud de registro y el bypass de pre-registration. La tarea se considera terminada cuando el escenario pasa y las credenciales devueltas se persisten.