test(batch55): port 75 NoRace integration tests

Ports 51 tests from norace_1_test.go and 24 tests from norace_2_test.go
as [SkippableFact] integration tests. Creates test harness infrastructure
(IntegrationTestBase, CheckHelper, NatsTestClient, TestServerHelper,
TestCluster) and tags all tests with [Trait("Category", "NoRace")].
Tests skip unless NATS_INTEGRATION_ENABLED=true is set.

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

@@ -0,0 +1,46 @@
+// Copyright 2012-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+using NATS.Client.Core;
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Helper for creating NATS client connections in integration tests.
+/// Wraps NATS.Client.Core connection setup.
+/// </summary>
+public sealed class NatsTestClient : IAsyncDisposable
+{
+    private readonly NatsConnection _connection;
+
+    private NatsTestClient(NatsConnection connection)
+    {
+        _connection = connection;
+    }
+
+    public NatsConnection Connection => _connection;
+
+    /// <summary>
+    /// Connects to a NATS server at the given URL.
+    /// </summary>
+    public static async Task<NatsTestClient> Connect(string url)
+    {
+        var opts = new NatsOpts { Url = url };
+        var conn = new NatsConnection(opts);
+        await conn.ConnectAsync();
+        return new NatsTestClient(conn);
+    }
+
+    /// <summary>
+    /// Connects to the local NATS server at its default port (for stub tests).
+    /// </summary>
+    public static async Task<NatsTestClient> ConnectToServer(string serverUrl)
+    {
+        return await Connect(serverUrl);
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _connection.DisposeAsync();
+    }
+}