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/IntegrationTestBase.cs

@@ -0,0 +1,57 @@
+// 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.
+
+using Xunit.Abstractions;
+
+namespace ZB.MOM.NatsNet.Server.IntegrationTests;
+
+/// <summary>
+/// Abstract base class for all integration tests.
+/// Skips the entire test class if the server cannot boot (i.e., the .NET server
+/// runtime is not yet complete). Individual test classes inherit from this class.
+/// </summary>
+[Trait("Category", "Integration")]
+public abstract class IntegrationTestBase : IDisposable
+{
+    // =========================================================================
+    // Constructor — Skip guard
+    // =========================================================================
+
+    /// <summary>
+    /// Initializes the test base and verifies that the server can boot.
+    /// If <see cref="Helpers.TestServerHelper.CanBoot()"/> returns false the test
+    /// is skipped via <c>Xunit.SkippableFact</c>'s <c>Skip.If</c> mechanism.
+    /// </summary>
+    protected IntegrationTestBase(ITestOutputHelper output)
+    {
+        Output = output;
+        Skip.If(!Helpers.TestServerHelper.CanBoot(), "Server cannot boot — skipping integration tests.");
+    }
+
+    // =========================================================================
+    // Protected members
+    // =========================================================================
+
+    /// <summary>xUnit output helper, available to derived test classes.</summary>
+    protected ITestOutputHelper Output { get; }
+
+    // =========================================================================
+    // IDisposable
+    // =========================================================================
+
+    /// <summary>
+    /// Override in subclasses to perform per-test cleanup (e.g., shut down servers,
+    /// delete temp dirs). The base implementation does nothing.
+    /// </summary>
+    public virtual void Dispose() { }
+}