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/IntegrationTestBase.cs

@@ -0,0 +1,39 @@
+// Copyright 2012-2025 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 running NATS server or cluster.
+/// Tests derived from this class are skipped when the infrastructure is not available.
+/// These tests correspond to Go's !race build tag tests (TestNoRace* functions).
+/// </summary>
+[Trait("Category", "Integration")]
+public abstract class IntegrationTestBase
+{
+    protected readonly ITestOutputHelper Output;
+
+    /// <summary>
+    /// Set to true when the NATS_INTEGRATION_ENABLED environment variable is set to "true".
+    /// When false, all [SkippableFact] tests will be skipped with an appropriate message.
+    /// </summary>
+    public static readonly bool IntegrationEnabled =
+        string.Equals(
+            Environment.GetEnvironmentVariable("NATS_INTEGRATION_ENABLED"),
+            "true",
+            StringComparison.OrdinalIgnoreCase);
+
+    protected IntegrationTestBase(ITestOutputHelper output)
+    {
+        Output = output;
+    }
+
+    /// <summary>
+    /// Returns a skip message when integration tests are not enabled.
+    /// Use with [SkippableFact]: throw new SkipException(SkipMessage) if !IntegrationEnabled.
+    /// </summary>
+    public static string SkipMessage =>
+        "Integration tests require NATS_INTEGRATION_ENABLED=true and a running NATS server/cluster";
+}