test(batch56): port 66 reload and auth integration tests

Port config hot-reload (44 tests), opts (1 test), account isolation
(5 tests), auth callout (5 tests), and JWT validation (11 tests) from
Go reload_test.go, opts_test.go, accounts_test.go, auth_callout_test.go,
and jwt_test.go as behavioral blackbox integration tests against the
.NET NatsServer using ReloadOptions() and the public NATS client API.

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

@@ -0,0 +1,117 @@
+// Copyright 2012-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Mirrors Go checkFor from server/test_test.go.
+
+using System.Diagnostics;
+using ZB.MOM.NatsNet.Server;
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests.Helpers;
+
+/// <summary>
+/// Retry/polling helpers for integration tests.
+/// Mirrors Go <c>checkFor</c> from server/test_test.go.
+/// </summary>
+internal static class CheckHelper
+{
+    /// <summary>
+    /// Polls <paramref name="check"/> repeatedly until it returns null (success)
+    /// or the timeout expires, in which case the last exception is thrown.
+    /// Mirrors Go <c>checkFor(t, timeout, interval, func() error)</c>.
+    /// </summary>
+    public static void CheckFor(TimeSpan timeout, TimeSpan interval, Func<Exception?> check)
+    {
+        var sw = Stopwatch.StartNew();
+        Exception? last = null;
+        while (sw.Elapsed < timeout)
+        {
+            last = check();
+            if (last == null) return;
+            Thread.Sleep(interval);
+        }
+
+        // One final attempt after the sleep boundary.
+        last = check();
+        if (last == null) return;
+
+        throw new TimeoutException(
+            $"CheckFor timed out after {timeout}: {last.Message}", last);
+    }
+
+    /// <summary>
+    /// Async version of <see cref="CheckFor"/>. Uses <c>Task.Delay</c> instead of
+    /// <c>Thread.Sleep</c> to avoid blocking the thread pool.
+    /// </summary>
+    public static async Task CheckForAsync(
+        TimeSpan timeout,
+        TimeSpan interval,
+        Func<Task<Exception?>> check,
+        CancellationToken cancellationToken = default)
+    {
+        var sw = Stopwatch.StartNew();
+        Exception? last = null;
+        while (sw.Elapsed < timeout)
+        {
+            last = await check().ConfigureAwait(false);
+            if (last == null) return;
+            await Task.Delay(interval, cancellationToken).ConfigureAwait(false);
+        }
+
+        // One final attempt.
+        last = await check().ConfigureAwait(false);
+        if (last == null) return;
+
+        throw new TimeoutException(
+            $"CheckForAsync timed out after {timeout}: {last.Message}", last);
+    }
+
+    /// <summary>
+    /// Waits until all servers in <paramref name="servers"/> have formed a cluster
+    /// (each server sees at least <c>servers.Length - 1</c> routes).
+    /// Uses a 10-second timeout with 100 ms poll interval.
+    /// Mirrors Go <c>checkClusterFormed</c>.
+    /// </summary>
+    public static void CheckClusterFormed(params NatsServer[] servers)
+    {
+        var expected = servers.Length - 1;
+        CheckFor(TimeSpan.FromSeconds(10), TimeSpan.FromMilliseconds(100), () =>
+        {
+            foreach (var s in servers)
+            {
+                var routes = s.NumRoutes();
+                if (routes < expected)
+                    return new Exception(
+                        $"Server {s.Options.ServerName} has {routes} routes, expected {expected}.");
+            }
+            return null;
+        });
+    }
+
+    /// <summary>
+    /// Waits until the given server has at least <paramref name="expected"/>
+    /// leaf node connections.
+    /// Uses a 10-second timeout with 100 ms poll interval.
+    /// Mirrors Go <c>checkLeafNodeConnectedCount</c>.
+    /// </summary>
+    public static void CheckLeafNodeConnectedCount(NatsServer server, int expected)
+    {
+        CheckFor(TimeSpan.FromSeconds(10), TimeSpan.FromMilliseconds(100), () =>
+        {
+            var count = server.NumLeafNodes();
+            if (count < expected)
+                return new Exception(
+                    $"Server {server.Options.ServerName} has {count} leaf nodes, expected {expected}.");
+            return null;
+        });
+    }
+}