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/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.");
+}