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/NoRespondersTests.cs

@@ -0,0 +1,99 @@
+using System.Net;
+using System.Net.Sockets;
+using System.Text;
+using Microsoft.Extensions.Logging.Abstractions;
+using NATS.Server;
+using NATS.Server.TestUtilities;
+
+namespace NATS.Server.Core.Tests;
+
+public class NoRespondersTests : IAsyncLifetime
+{
+    private readonly NatsServer _server;
+    private readonly int _port;
+    private readonly CancellationTokenSource _cts = new();
+
+    public NoRespondersTests()
+    {
+        _port = TestPortAllocator.GetFreePort();
+        _server = new NatsServer(new NatsOptions { Port = _port }, NullLoggerFactory.Instance);
+    }
+
+    public async Task InitializeAsync()
+    {
+        _ = _server.StartAsync(_cts.Token);
+        await _server.WaitForReadyAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _cts.CancelAsync();
+        _server.Dispose();
+    }
+
+
+    private async Task<Socket> ConnectClientAsync()
+    {
+        var sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
+        await sock.ConnectAsync(IPAddress.Loopback, _port);
+        return sock;
+    }
+
+
+    [Fact]
+    public async Task NoResponders_without_headers_closes_connection()
+    {
+        using var client = await ConnectClientAsync();
+
+        // Read INFO
+        await SocketTestHelper.ReadUntilAsync(client, "\r\n");
+
+        // Send CONNECT with no_responders:true but headers:false
+        await client.SendAsync(Encoding.ASCII.GetBytes(
+            "CONNECT {\"no_responders\":true,\"headers\":false}\r\n"));
+
+        // Should receive -ERR and connection should close
+        var response = await SocketTestHelper.ReadUntilAsync(client, "-ERR");
+        response.ShouldContain("-ERR");
+        response.ShouldContain("No Responders Requires Headers Support");
+    }
+
+    [Fact]
+    public async Task NoResponders_with_headers_accepted()
+    {
+        using var client = await ConnectClientAsync();
+
+        // Read INFO
+        await SocketTestHelper.ReadUntilAsync(client, "\r\n");
+
+        // Send CONNECT with both no_responders and headers true, then PING
+        await client.SendAsync(Encoding.ASCII.GetBytes(
+            "CONNECT {\"no_responders\":true,\"headers\":true}\r\nPING\r\n"));
+
+        // Should receive PONG (connection stays alive)
+        var response = await SocketTestHelper.ReadUntilAsync(client, "PONG\r\n");
+        response.ShouldContain("PONG\r\n");
+    }
+
+    [Fact]
+    public async Task NoResponders_sends_503_when_no_subscribers()
+    {
+        using var client = await ConnectClientAsync();
+
+        // Read INFO
+        await SocketTestHelper.ReadUntilAsync(client, "\r\n");
+
+        // CONNECT with no_responders and headers enabled
+        // SUB to the reply inbox so we can receive the 503
+        // PUB to a subject with no subscribers, using a reply-to subject
+        await client.SendAsync(Encoding.ASCII.GetBytes(
+            "CONNECT {\"no_responders\":true,\"headers\":true}\r\n" +
+            "SUB _INBOX.reply 1\r\n" +
+            "PUB no.subscribers _INBOX.reply 5\r\nHello\r\n"));
+
+        // Should receive HMSG with 503 status on the reply subject
+        var response = await SocketTestHelper.ReadUntilAsync(client, "HMSG");
+        response.ShouldContain("HMSG _INBOX.reply 1");
+        response.ShouldContain("503");
+    }
+}