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/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;
+}