refactor: rename remaining tests to NATS.Server.Core.Tests

- Rename tests/NATS.Server.Tests -> tests/NATS.Server.Core.Tests
- Update solution file, InternalsVisibleTo, and csproj references
- Remove JETSTREAM_INTEGRATION_MATRIX and NATS.NKeys from csproj (moved to JetStream.Tests and Auth.Tests)
- Update all namespaces from NATS.Server.Tests.* to NATS.Server.Core.Tests.*
- Replace private GetFreePort/ReadUntilAsync helpers with TestUtilities calls
- Fix stale namespace in Transport.Tests/NetworkingGoParityTests.cs

tests/NATS.Server.Core.Tests/Server/AcceptLoopErrorCallbackTests.cs

@@ -0,0 +1,41 @@
+using System.Net;
+using System.Net.Sockets;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server.Server;
+
+namespace NATS.Server.Core.Tests;
+
+public class AcceptLoopErrorCallbackTests
+{
+    [Fact]
+    public void Accept_loop_reports_error_via_callback_hook()
+    {
+        var server = new NatsServer(new NatsOptions
+        {
+            Host = "127.0.0.1",
+            Port = 0,
+        }, NullLoggerFactory.Instance);
+
+        Exception? capturedError = null;
+        EndPoint? capturedEndpoint = null;
+        var capturedDelay = TimeSpan.Zero;
+
+        var handler = new AcceptLoopErrorHandler((ex, endpoint, delay) =>
+        {
+            capturedError = ex;
+            capturedEndpoint = endpoint;
+            capturedDelay = delay;
+        });
+
+        server.SetAcceptLoopErrorHandlerForTest(handler);
+
+        var endpoint = new IPEndPoint(IPAddress.Loopback, 4222);
+        var error = new SocketException((int)SocketError.ConnectionReset);
+        var delay = TimeSpan.FromMilliseconds(20);
+        server.NotifyAcceptErrorForTest(error, endpoint, delay);
+
+        capturedError.ShouldBe(error);
+        capturedEndpoint.ShouldBe(endpoint);
+        capturedDelay.ShouldBe(delay);
+    }
+}