Fix NKey nonce verification: the NATS client signs the nonce string
(ASCII bytes of the base64url-encoded nonce), not the raw nonce bytes.
Pass the encoded nonce string bytes to the authenticator for verification.
Subscriptions and message routing now go through account-specific SubLists
instead of a single global SubList. Clients in different accounts cannot
see each other's messages. When no account is specified (or auth is not
configured), all clients share the global $G account.
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.
Replace empty catch blocks with meaningful log statements in NatsServer,
NatsClient, and Program. Add WaitForReadyAsync() to NatsServer for
deterministic server startup. Replace Task.Delay/Thread.Sleep in tests
with PING/PONG protocol flush and SubscribeCoreAsync for reliable
subscription synchronization.
