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/TestCluster.cs

@@ -0,0 +1,75 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Represents a test JetStream cluster. In the current integration test setup,
+/// tests that use this type are skipped unless a real cluster is available.
+/// This is a scaffold/stub that captures the API shape for future full implementation.
+/// </summary>
+public sealed class TestCluster : IDisposable
+{
+    private readonly string _name;
+    private readonly int _numServers;
+    private bool _disposed;
+
+    public string Name => _name;
+
+    private TestCluster(string name, int numServers)
+    {
+        _name = name;
+        _numServers = numServers;
+    }
+
+    /// <summary>
+    /// Creates a JetStream cluster with the given number of servers.
+    /// Throws <see cref="ClusterNotAvailableException"/> if no cluster is available.
+    /// </summary>
+    public static TestCluster CreateJetStreamCluster(int numServers, string name = "JSC")
+    {
+        throw new ClusterNotAvailableException(
+            $"No JetStream cluster available. Set NATS_INTEGRATION_TESTS=true and ensure {numServers}-node cluster '{name}' is running.");
+    }
+
+    /// <summary>
+    /// Creates a JetStream cluster with the specified configuration template.
+    /// </summary>
+    public static TestCluster CreateJetStreamClusterWithTemplate(string template, int numServers, string name = "JSC")
+    {
+        throw new ClusterNotAvailableException(
+            $"No JetStream cluster available for template cluster '{name}'.");
+    }
+
+    public void WaitOnClusterReady() { }
+    public void WaitOnLeader() { }
+    public void WaitOnStreamLeader(string account, string stream) { }
+    public void WaitOnConsumerLeader(string account, string stream, string consumer) { }
+    public void WaitOnPeerCount(int count) { }
+    public void WaitOnServerCurrent(object server) { }
+    public void WaitOnStreamCurrent(object server, string account, string stream) { }
+
+    public object Leader() => throw new ClusterNotAvailableException("Cluster not available.");
+    public object RandomServer() => throw new ClusterNotAvailableException("Cluster not available.");
+    public object RandomNonLeader() => throw new ClusterNotAvailableException("Cluster not available.");
+    public object RandomNonStreamLeader(string account, string stream) => throw new ClusterNotAvailableException("Cluster not available.");
+    public object ServerByName(string name) => throw new ClusterNotAvailableException("Cluster not available.");
+    public object StreamLeader(string account, string stream) => throw new ClusterNotAvailableException("Cluster not available.");
+    public object ConsumerLeader(string account, string stream, string consumer) => throw new ClusterNotAvailableException("Cluster not available.");
+
+    public void Dispose()
+    {
+        if (!_disposed)
+        {
+            _disposed = true;
+        }
+    }
+}
+
+/// <summary>
+/// Thrown when a test cluster is not available for integration testing.
+/// </summary>
+public sealed class ClusterNotAvailableException : Exception
+{
+    public ClusterNotAvailableException(string message) : base(message) { }
+}