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/CheckHelper.cs

@@ -0,0 +1,39 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Polling / assertion helpers ported from the Go test framework's checkFor and
+/// related utilities.
+/// </summary>
+public static class CheckHelper
+{
+    /// <summary>
+    /// Polls <paramref name="check"/> every <paramref name="interval"/> until it
+    /// returns null (success) or <paramref name="timeout"/> expires.
+    /// </summary>
+    public static void CheckFor(
+        TimeSpan timeout,
+        TimeSpan interval,
+        Func<string?> check)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        string? last = null;
+        while (DateTime.UtcNow < deadline)
+        {
+            last = check();
+            if (last is null)
+                return;
+            Thread.Sleep(interval);
+        }
+        throw new Xunit.Sdk.XunitException($"CheckFor timed out after {timeout}: {last}");
+    }
+
+    /// <summary>
+    /// Waits until the supplied server reports exactly <paramref name="expected"/>
+    /// leaf-node connections.
+    /// </summary>
+    public static void CheckLeafNodeConnectedCount(object server, int expected) =>
+        throw new NotImplementedException("Requires a running server instance.");
+}

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

@@ -0,0 +1,38 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Helpers for generating and writing NATS server configuration files used by
+/// integration tests.
+/// </summary>
+public static class ConfigHelper
+{
+    /// <summary>
+    /// Template for a basic JetStream super-cluster node.
+    /// Placeholders: {ServerName}, {ClusterName}, {StoreDir}, {ClusterPort}, {Routes}
+    /// </summary>
+    public const string JsSuperClusterTemplate = """
+        listen: 127.0.0.1:-1
+        server_name: {ServerName}
+        jetstream: {max_mem_store: 256MB, max_file_store: 2GB, store_dir: '{StoreDir}'}
+        cluster {
+            name: {ClusterName}
+            listen: 127.0.0.1:{ClusterPort}
+            routes = [{Routes}]
+        }
+        accounts { $SYS { users = [ { user: "admin", pass: "s3cr3t!" } ] } }
+        """;
+
+    /// <summary>
+    /// Writes <paramref name="content"/> to a temporary file and returns its path.
+    /// The file is deleted when the process exits (via <see cref="Path.GetTempFileName"/>).
+    /// </summary>
+    public static string CreateConfigFile(string content)
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"natsnet_{Guid.NewGuid():N}.conf");
+        File.WriteAllText(path, content);
+        return path;
+    }
+}

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

@@ -0,0 +1,18 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Base class for integration tests that require a running .NET NatsServer.
+/// Tests that depend on multi-server cluster infrastructure skip automatically
+/// until the full server runtime is available.
+/// </summary>
+public abstract class IntegrationTestBase
+{
+    /// <summary>
+    /// Returns true when the full server runtime required by cluster / super-cluster
+    /// tests is not yet available.
+    /// </summary>
+    protected static bool ServerRuntimeUnavailable => true;
+}

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

@@ -0,0 +1,24 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+using NATS.Client.Core;
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Helpers for connecting NATS test clients to running servers.
+/// </summary>
+public static class NatsTestClient
+{
+    /// <summary>Connects to the given NATS URL.</summary>
+    public static async Task<NatsConnection> Connect(string url)
+    {
+        var conn = new NatsConnection(new NatsOpts { Url = url });
+        await conn.ConnectAsync();
+        return conn;
+    }
+
+    /// <summary>Connects to the client URL of the supplied server handle.</summary>
+    public static Task<NatsConnection> ConnectToServer(object server) =>
+        throw new NotImplementedException("Requires a running server instance.");
+}

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.
+    }
+}

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

@@ -0,0 +1,18 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Helpers for starting individual NATS server instances in integration tests.
+/// </summary>
+public static class TestServerHelper
+{
+    /// <summary>Runs a server with programmatically supplied options.</summary>
+    public static IDisposable RunServer(object opts) =>
+        throw new NotImplementedException("Requires full NatsServer runtime.");
+
+    /// <summary>Runs a server from a config file on disk.</summary>
+    public static IDisposable RunServerWithConfig(string configFilePath) =>
+        throw new NotImplementedException("Requires full NatsServer 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();
+    }
+}