[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

Start with the AS metadata work in #318, then inspect Mcp\Client\Auth\DynamicClientRegistrar, Client\Builder, and TokenStorageInterface. Run the auth/pre-registration conformance scenario and add the requested unit tests for registration request shape and pre-registration bypass. Done means the scenario passes and returned credentials are persisted.