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/ConfigHelper.cs

@@ -0,0 +1,66 @@
+// Copyright 2020-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Provides configuration templates and file helpers for integration tests.
+/// Mirrors Go's <c>jsClusterTempl</c>, <c>jsClusterAccountsTempl</c>, etc.
+/// </summary>
+public static class ConfigHelper
+{
+    /// <summary>
+    /// Standard JetStream cluster configuration template.
+    /// Use <c>%s</c> placeholders for server_name, store_dir, cluster_name, port, and routes.
+    /// </summary>
+    public const string JsClusterTemplate = @"
+listen: 127.0.0.1:-1
+server_name: %s
+jetstream: {max_mem_store: 256MB, max_file_store: 2GB, store_dir: '%s'}
+
+cluster {
+    name: %s
+    listen: 127.0.0.1:%d
+    routes = [%s]
+}
+";
+
+    /// <summary>
+    /// JetStream cluster template with accounts.
+    /// </summary>
+    public const string JsClusterAccountsTemplate = @"
+listen: 127.0.0.1:-1
+server_name: %s
+jetstream: {max_mem_store: 256MB, max_file_store: 2GB, store_dir: '%s'}
+
+accounts {
+    $SYS {
+        users: [{user: admin, password: s3cr3t!}]
+    }
+    ONE {
+        jetstream: enabled
+        users: [{user: one, password: p}]
+    }
+    TWO {
+        jetstream: enabled
+        users: [{user: two, password: p}]
+    }
+}
+
+cluster {
+    name: %s
+    listen: 127.0.0.1:%d
+    routes = [%s]
+}
+";
+
+    /// <summary>
+    /// Creates a temporary config file with the given content and returns its path.
+    /// </summary>
+    public static string CreateConfigFile(string content)
+    {
+        var path = Path.Combine(Path.GetTempPath(), $"nats-test-{Guid.NewGuid():N}.conf");
+        File.WriteAllText(path, content);
+        return path;
+    }
+}