Add TlsMixedModeTests verifying that a server with AllowNonTls=true
accepts both plaintext and TLS clients on the same port. Add
MonitorTlsTests verifying that /connz reports TlsVersion and
TlsCipherSuite for TLS-connected clients.
Integrate TLS support into the server's connection accept path:
- Add SslServerAuthenticationOptions and TlsRateLimiter fields to NatsServer
- Extract AcceptClientAsync method for TLS negotiation, rate limiting, and
TLS state extraction (protocol version, cipher suite, peer certificate)
- Add InfoAlreadySent flag to NatsClient to skip redundant INFO when
TlsConnectionWrapper already sent it during negotiation
- Add TlsServerTests verifying TLS connect+INFO and TLS pub/sub
