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/CheckHelper.cs

@@ -0,0 +1,39 @@
+// Copyright 2012-2025 The NATS Authors
+// Licensed under the Apache License, Version 2.0
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Helper for polling-style checks in integration tests.
+/// Corresponds to Go's checkFor(t, timeout, interval, func) pattern.
+/// </summary>
+public static class CheckHelper
+{
+    /// <summary>
+    /// Polls the provided check function until it returns null (success) or the timeout elapses.
+    /// Throws an exception with the last error message if the timeout is reached.
+    /// </summary>
+    /// <param name="timeout">Maximum time to wait.</param>
+    /// <param name="interval">Polling interval.</param>
+    /// <param name="check">Function returning null on success, or an error message on failure.</param>
+    public static async Task CheckFor(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 == null) return;
+            await Task.Delay(interval);
+        }
+        throw new InvalidOperationException($"CheckFor timed out after {timeout}: {lastError}");
+    }
+
+    /// <summary>
+    /// Polls the provided synchronous check function until it returns null (success) or the timeout elapses.
+    /// </summary>
+    public static async Task CheckFor(TimeSpan timeout, TimeSpan interval, Func<string?> check)
+    {
+        await CheckFor(timeout, interval, () => Task.FromResult(check()));
+    }
+}