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/TestCluster.cs

@@ -0,0 +1,29 @@
+// 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 single JetStream cluster (multiple servers forming a
+/// Raft group).  The real implementation requires the full NatsServer runtime.
+/// </summary>
+public sealed class TestCluster : IDisposable
+{
+    public string Name { get; }
+
+    public TestCluster(string name)
+    {
+        Name = name;
+    }
+
+    /// <summary>
+    /// Creates a JetStream cluster.  Not yet implemented.
+    /// </summary>
+    public static TestCluster CreateJetStreamCluster(int numServers, string name) =>
+        new(name);
+
+    public void Dispose()
+    {
+        // TODO: shut down all in-process servers when runtime is available.
+    }
+}