feat: enhance ConfigProcessor & add 110 Go-parity opts tests (Task 22)

Port configuration parsing for NKey users, gateway remotes, leaf node
remotes, auth timeout, write_deadline, websocket ping_interval, and
token+users conflict validation. Add RemoteGatewayOptions, enhanced
LeafNodeOptions with remotes support.

110 new tests ported from opts_test.go.

src/NATS.Server/Configuration/ClusterOptions.cs

@@ -9,4 +9,7 @@ public sealed class ClusterOptions
     public List<string> Routes { get; set; } = [];
     public List<string> Accounts { get; set; } = [];
     public RouteCompression Compression { get; set; } = RouteCompression.None;
+
+    // Go: opts.go — cluster write_deadline
+    public TimeSpan WriteDeadline { get; set; }
 }