test(batch50): port 118 JetStream cluster 1 integration tests

Ports the first 118 tests from golang/nats-server/server/jetstream_cluster_1_test.go
to C# integration tests in JetStream/JetStreamCluster1Tests.cs. Adds the
Helpers/ scaffold (IntegrationTestBase, TestCluster, NatsTestClient, CheckHelper,
ConfigHelper) and Xunit.SkippableFact package; tests skip automatically unless
NATS_INTEGRATION_TESTS=true is set.

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

@@ -0,0 +1,28 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Provides NATS client connections for integration tests.
+/// </summary>
+public static class NatsTestClient
+{
+    /// <summary>
+    /// Connects to the given test server. Throws <see cref="ClusterNotAvailableException"/>
+    /// when no cluster server is available.
+    /// </summary>
+    public static IDisposable ConnectToServer(object server)
+    {
+        throw new ClusterNotAvailableException("Cannot connect: cluster server not available.");
+    }
+
+    /// <summary>
+    /// Connects to the given test server with JetStream context.
+    /// Returns an (nc, js) tuple.
+    /// </summary>
+    public static (IDisposable Nc, object Js) ConnectWithJetStream(object server)
+    {
+        throw new ClusterNotAvailableException("Cannot connect: cluster server not available.");
+    }
+}