refactor: extract NATS.Server.Mqtt.Tests project

Move 29 MQTT test files from NATS.Server.Tests into a dedicated
NATS.Server.Mqtt.Tests project. Update namespaces, add
InternalsVisibleTo, and replace Task.Delay calls with
PollHelper.WaitUntilAsync for proper synchronization.

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttAdvancedParityTests.cs

@@ -12,8 +12,9 @@ using System.Net;
 using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
+using NATS.Server.TestUtilities;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttAdvancedParityTests
 {
@@ -763,10 +764,22 @@ public class MqttAdvancedParityTests
         // Disconnect
         client.Dispose();
 
-        // Server should not crash
-        await Task.Delay(100);
+        // Verify server is still operational by polling until it accepts a new connection
+        var connected = await PollHelper.WaitUntilAsync(async () =>
+        {
+            try
+            {
+                using var probe = new TcpClient();
+                await probe.ConnectAsync(IPAddress.Loopback, listener.Port);
+                return true;
+            }
+            catch
+            {
+                return false;
+            }
+        }, timeoutMs: 2000, intervalMs: 10);
+        connected.ShouldBeTrue("Server should still accept connections after client disconnect");
 
-        // Verify server is still operational
         using var client2 = new TcpClient();
         await client2.ConnectAsync(IPAddress.Loopback, listener.Port);
         var s2 = client2.GetStream();

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttAuthIntegrationTests.cs

@@ -2,7 +2,7 @@ using System.Net;
 using System.Net.Sockets;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttAuthIntegrationTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttAuthParityTests.cs

@@ -9,8 +9,9 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Auth;
 using NATS.Server.Mqtt;
+using NATS.Server.TestUtilities;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttAuthParityTests
 {
@@ -202,10 +203,13 @@ public class MqttAuthParityTests
         await MqttAuthWire.WriteLineAsync(stream, "CONNECT keepalive-client clean=true keepalive=1");
         (await MqttAuthWire.ReadLineAsync(stream, 1000)).ShouldBe("CONNACK");
 
-        await Task.Delay(2500);
-
-        var result = await MqttAuthWire.ReadRawAsync(stream, 500);
-        (result == null || result == "__timeout__").ShouldBeTrue();
+        // Poll until the server closes the connection due to keepalive expiry (keepalive=1s)
+        var disconnected = await PollHelper.WaitUntilAsync(async () =>
+        {
+            var result = await MqttAuthWire.ReadRawAsync(stream, 200);
+            return result == null || result == "__timeout__";
+        }, timeoutMs: 5000, intervalMs: 100);
+        disconnected.ShouldBeTrue("Server should disconnect idle client after keepalive timeout");
     }
 
     // =========================================================================

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttBinaryParserTests.cs

@@ -8,7 +8,7 @@
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttBinaryParserTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttFlapperDetectionTests.cs

@@ -4,7 +4,7 @@
 using NATS.Server.Mqtt;
 using Shouldly;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttFlapperDetectionTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttFlowControllerTests.cs

@@ -3,7 +3,7 @@
 using NATS.Server.Mqtt;
 using Shouldly;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public sealed class MqttFlowControllerTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttGoParityTests.cs

@@ -8,7 +8,7 @@
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 /// <summary>
 /// Parity tests ported from Go server/mqtt_test.go exercising MQTT binary

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttKeepAliveTests.cs

@@ -1,8 +1,9 @@
 using System.Net;
 using System.Net.Sockets;
 using NATS.Server.Mqtt;
+using NATS.Server.TestUtilities;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttKeepAliveTests
 {
@@ -20,7 +21,12 @@ public class MqttKeepAliveTests
         await MqttRuntimeWire.WriteLineAsync(stream, "CONNECT keepalive-client keepalive=1");
         (await MqttRuntimeWire.ReadLineAsync(stream, 1000)).ShouldBe("CONNACK");
 
-        await Task.Delay(2000);
-        (await MqttRuntimeWire.ReadRawAsync(stream, 1000)).ShouldBeNull();
+        // Poll until the server closes the connection due to keepalive expiry (keepalive=1s)
+        var disconnected = await PollHelper.WaitUntilAsync(async () =>
+        {
+            var result = await MqttRuntimeWire.ReadRawAsync(stream, 200);
+            return result == null;
+        }, timeoutMs: 5000, intervalMs: 100);
+        disconnected.ShouldBeTrue("Server should disconnect idle client after keepalive timeout");
     }
 }

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttListenerParityTests.cs

@@ -3,7 +3,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests;
+namespace NATS.Server.Mqtt.Tests;
 
 public class MqttListenerParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttModelParityBatch3Tests.cs

@@ -1,6 +1,6 @@
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttModelParityBatch3Tests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttPacketParserTests.cs

@@ -1,6 +1,6 @@
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttPacketParserTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttPacketParsingParityTests.cs

@@ -5,7 +5,7 @@
 
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttPacketParsingParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttPacketWriterTests.cs

@@ -1,6 +1,6 @@
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttPacketWriterTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttProtocolConstantsParityBatch1Tests.cs

@@ -1,7 +1,7 @@
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttProtocolConstantsParityBatch1Tests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttProtocolConstantsParityBatch2Tests.cs

@@ -1,7 +1,7 @@
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttProtocolConstantsParityBatch2Tests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttPublishSubscribeParityTests.cs

@@ -2,7 +2,7 @@ using System.Net;
 using System.Net.Sockets;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests;
+namespace NATS.Server.Mqtt.Tests;
 
 public class MqttPublishSubscribeParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttQoSTrackingTests.cs

@@ -4,7 +4,7 @@
 using NATS.Server.Mqtt;
 using Shouldly;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public sealed class MqttQoSTrackingTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttQosAckRuntimeTests.cs

@@ -2,7 +2,7 @@ using System.Net;
 using System.Net.Sockets;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttQosAckRuntimeTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttQosDeliveryParityTests.cs

@@ -6,7 +6,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttQosDeliveryParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttQosTests.cs

@@ -6,7 +6,7 @@
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttQosTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttRetainedDeliveryTests.cs

@@ -6,7 +6,7 @@ using System.Text;
 using NATS.Server.Mqtt;
 using Shouldly;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttRetainedDeliveryTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttRetainedMessageParityTests.cs

@@ -8,7 +8,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttRetainedMessageParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttSessionParityTests.cs

@@ -7,7 +7,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttSessionParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttSessionPersistenceTests.cs

@@ -5,7 +5,7 @@
 
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttSessionPersistenceTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttSessionRuntimeTests.cs

@@ -3,7 +3,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttSessionRuntimeTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttTopicMappingParityTests.cs

@@ -4,7 +4,7 @@
 // golang/nats-server/server/mqtt.go — mqttTopicToNATSPubSubject, mqttFilterToNATSSubject,
 // natsSubjectToMQTTTopic, mqttToNATSSubjectConversion
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 /// <summary>
 /// Tests MQTT topic to NATS subject conversion and vice versa, porting the

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttWillMessageParityTests.cs

@@ -7,7 +7,7 @@ using System.Net.Sockets;
 using System.Text;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttWillMessageParityTests
 {

tests/NATS.Server.Mqtt.Tests/Mqtt/MqttWillMessageTests.cs

@@ -5,7 +5,7 @@
 using NATS.Server.Mqtt;
 using Shouldly;
 
-namespace NATS.Server.Tests.Mqtt;
+namespace NATS.Server.Mqtt.Tests.Mqtt;
 
 public class MqttWillMessageTests
 {

tests/NATS.Server.Mqtt.Tests/MqttPersistenceTests.cs

@@ -2,7 +2,7 @@ using NSubstitute;
 using NATS.Server.JetStream.Storage;
 using NATS.Server.Mqtt;
 
-namespace NATS.Server.Tests;
+namespace NATS.Server.Mqtt.Tests;
 
 // Go reference: server/mqtt.go ($MQTT_msgs, $MQTT_sess, $MQTT_rmsgs JetStream streams)
 

tests/NATS.Server.Mqtt.Tests/NATS.Server.Mqtt.Tests.csproj

@@ -0,0 +1,25 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="coverlet.collector" />
+    <PackageReference Include="Microsoft.NET.Test.Sdk" />
+    <PackageReference Include="NATS.Client.Core" />
+    <PackageReference Include="NSubstitute" />
+    <PackageReference Include="Shouldly" />
+    <PackageReference Include="xunit" />
+    <PackageReference Include="xunit.runner.visualstudio" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <Using Include="Xunit" />
+    <Using Include="Shouldly" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\NATS.Server\NATS.Server.csproj" />
+    <ProjectReference Include="..\NATS.Server.TestUtilities\NATS.Server.TestUtilities.csproj" />
+  </ItemGroup>
+</Project>