[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

#318 の AS メタデータ作業から始め、次に Mcp\Client\Auth\DynamicClientRegistrar、Client\Builder、TokenStorageInterface を調査します。auth/pre-registration の適合性シナリオを実行し、登録リクエストの構造と事前登録のバイパスについて要求された単体テストを追加します。シナリオが成功し、返された認証情報が永続化されれば完了です。