test(batch57): port 53 SuperCluster and LeafNode integration tests

Ports 36 JetStream super-cluster tests from jetstream_super_cluster_test.go,
3 JetStream leaf-node tests from jetstream_leafnode_test.go, and 14 leaf-node
tests from leafnode_test.go into the integration test project. Creates the
required harness infrastructure (TestSuperCluster, TestCluster, IntegrationTestBase,
CheckHelper, ConfigHelper, NatsTestClient, TestServerHelper). All 53 tests are
marked [Fact(Skip = "...")] pending full multi-server cluster runtime.

dotnet/tests/ZB.MOM.NatsNet.Server.IntegrationTests/Helpers/TestSuperCluster.cs

@@ -0,0 +1,49 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Placeholder harness for a JetStream super-cluster (multiple clusters connected
+/// by gateways).  The real implementation requires the full NatsServer runtime to
+/// be capable of spawning in-process cluster nodes.
+/// </summary>
+public sealed class TestSuperCluster : IDisposable
+{
+    private readonly List<TestCluster> _clusters;
+
+    private TestSuperCluster(List<TestCluster> clusters)
+    {
+        _clusters = clusters;
+    }
+
+    /// <summary>
+    /// Creates a super-cluster with <paramref name="numClusters"/> clusters, each
+    /// containing <paramref name="numPerCluster"/> servers.
+    /// Not yet implemented — tests that call this will be skipped.
+    /// </summary>
+    public static TestSuperCluster CreateJetStreamSuperCluster(int numPerCluster, int numClusters) =>
+        new([]);
+
+    /// <summary>Returns the current JetStream meta-leader across all clusters.</summary>
+    public object? Leader() => null;
+
+    /// <summary>Returns a random server from any cluster in the super-cluster.</summary>
+    public object? RandomServer() => null;
+
+    /// <summary>Waits until a meta-leader is elected.</summary>
+    public void WaitOnLeader() { /* placeholder */ }
+
+    /// <summary>Waits until a stream leader is elected in the given account.</summary>
+    public void WaitOnStreamLeader(string account, string stream) { /* placeholder */ }
+
+    /// <summary>Returns the named cluster.</summary>
+    public TestCluster ClusterForName(string name) =>
+        _clusters.Single(c => c.Name == name);
+
+    public void Dispose()
+    {
+        foreach (var c in _clusters)
+            c.Dispose();
+    }
+}