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/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;
+    }
+}