Wire AuthService into NatsServer and NatsClient to enforce authentication
on incoming connections. The server builds an AuthService from NatsOptions,
sets auth_required in ServerInfo, and generates per-client nonces when
NKey auth is configured. NatsClient validates credentials in ProcessConnect,
enforces publish/subscribe permissions, and implements an auth timeout that
closes connections that don't send CONNECT in time. Existing tests without
auth continue to work since AuthService.IsAuthRequired is false by default.
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