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/IntegrationTestBase.cs

@@ -0,0 +1,41 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+using Xunit.Abstractions;
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Base class for integration tests that require a live NATS cluster.
+/// Tests are skipped when the cluster infrastructure is not available.
+/// </summary>
+[Trait("Category", "Integration")]
+public abstract class IntegrationTestBase
+{
+    protected readonly ITestOutputHelper Output;
+
+    protected IntegrationTestBase(ITestOutputHelper output)
+    {
+        Output = output;
+    }
+
+    /// <summary>
+    /// Determines whether cluster integration tests should be skipped.
+    /// Tests are skipped unless the NATS_INTEGRATION_TESTS environment variable is set to "true"
+    /// or NATS_SERVER_URL is set to a valid cluster endpoint.
+    /// </summary>
+    protected static bool ShouldSkip()
+    {
+        var integrationEnabled = Environment.GetEnvironmentVariable("NATS_INTEGRATION_TESTS");
+        if (string.Equals(integrationEnabled, "true", StringComparison.OrdinalIgnoreCase))
+            return false;
+
+        var serverUrl = Environment.GetEnvironmentVariable("NATS_SERVER_URL");
+        if (!string.IsNullOrEmpty(serverUrl))
+            return false;
+
+        return true;
+    }
+
+    protected void Log(string message) => Output.WriteLine(message);
+}