feat: session A — config binding via appsettings.json (67 stubs complete)

Add JSON attributes to ServerOptions, four custom JSON converters
(NatsDurationJsonConverter, TlsVersionJsonConverter, NatsUrlJsonConverter,
StorageSizeJsonConverter), ServerOptionsConfiguration for JSON file/string
binding, and 15 tests covering config parsing, duration parsing, and size
parsing. Mark 67 opts.go features complete in porting.db.

dotnet/src/ZB.MOM.NatsNet.Server/ServerConstants.cs

@@ -63,6 +63,12 @@ public static class ServerConstants
     // Auth timeout — mirrors AUTH_TIMEOUT.
     public static readonly TimeSpan AuthTimeout = TimeSpan.FromSeconds(2);
 
+    // Default auth timeout as a double (seconds) — used by ServerOptions.AuthTimeout.
+    public const double DefaultAuthTimeout = 2.0;
+
+    // Maximum payload size alias used by config binding — mirrors MAX_PAYLOAD_SIZE.
+    public const int MaxPayload = MaxPayloadSize;
+
     // How often pings are sent — mirrors DEFAULT_PING_INTERVAL.
     public static readonly TimeSpan DefaultPingInterval = TimeSpan.FromMinutes(2);