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.");
+}