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/CheckHelper.cs

@@ -0,0 +1,46 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Provides polling-based check utilities for integration tests,
+/// mirroring the Go <c>checkFor</c> pattern.
+/// </summary>
+public static class CheckHelper
+{
+    /// <summary>
+    /// Polls <paramref name="check"/> every <paramref name="interval"/> until it returns
+    /// <c>null</c> (success) or <paramref name="timeout"/> elapses. Throws on timeout.
+    /// </summary>
+    public static void CheckFor(TimeSpan timeout, TimeSpan interval, Func<string?> check)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        string? lastError = null;
+        while (DateTime.UtcNow < deadline)
+        {
+            lastError = check();
+            if (lastError is null)
+                return;
+            Thread.Sleep(interval);
+        }
+        throw new TimeoutException($"CheckFor timed out after {timeout}: {lastError}");
+    }
+
+    /// <summary>
+    /// Async variant of <see cref="CheckFor"/>.
+    /// </summary>
+    public static async Task CheckForAsync(TimeSpan timeout, TimeSpan interval, Func<Task<string?>> check)
+    {
+        var deadline = DateTime.UtcNow + timeout;
+        string? lastError = null;
+        while (DateTime.UtcNow < deadline)
+        {
+            lastError = await check();
+            if (lastError is null)
+                return;
+            await Task.Delay(interval);
+        }
+        throw new TimeoutException($"CheckFor timed out after {timeout}: {lastError}");
+    }
+}