feat: add OcspMode enum, OcspConfig class, and wire into NatsOptions

Introduces NATS.Server.Tls.OcspMode (Auto/Always/Must/Never matching
Go ocsp.go constants) and OcspConfig with Mode and OverrideUrls. Adds
OcspConfig? and OcspPeerVerify to NatsOptions for stapling configuration
and peer certificate revocation checking. Covered by 12 new unit tests.

src/NATS.Server/NatsOptions.cs

@@ -1,5 +1,6 @@
 using System.Security.Authentication;
 using NATS.Server.Auth;
+using NATS.Server.Tls;
 
 namespace NATS.Server;
 
@@ -85,5 +86,9 @@ public sealed class NatsOptions
     public HashSet<string>? TlsPinnedCerts { get; set; }
     public SslProtocols TlsMinVersion { get; set; } = SslProtocols.Tls12;
 
+    // OCSP stapling and peer verification
+    public OcspConfig? OcspConfig { get; set; }
+    public bool OcspPeerVerify { get; set; }
+
     public bool HasTls => TlsCert != null && TlsKey != null;
 }