feat(mqtt): wire will message delivery on abnormal disconnect

Add DeliverWill to MqttPacketHandlers — extracts and clears the will,
then delivers via ProcessInboundClientMsg. Call from CloseConnection
when client is MQTT. Clean DISCONNECT already clears the will (Task 3),
so DeliverWill is a no-op after DISCONNECT. 3 new tests: abnormal
close delivers will, DISCONNECT suppresses will, no-will is a no-op.

dotnet/src/ZB.MOM.NatsNet.Server/ClientConnection.cs

@@ -1330,6 +1330,10 @@ public sealed partial class ClientConnection
             ClearPingTimer();
         }
 
+        // Deliver MQTT will message on abnormal disconnect.
+        if (IsMqtt())
+            MqttPacketHandlers.DeliverWill(this);
+
         // Close the underlying network connection.
         try { _nc?.Close(); } catch { /* ignore */ }
         _nc = null;

dotnet/src/ZB.MOM.NatsNet.Server/Mqtt/MqttPacketHandlers.cs

@@ -238,6 +238,33 @@ internal static class MqttPacketHandlers
         c.CloseConnection(ClosedState.ClientClosed);
     }
 
+    // =========================================================================
+    // Will message delivery
+    // =========================================================================
+
+    /// <summary>
+    /// Delivers the will message if one is set and the connection was not cleanly
+    /// disconnected (i.e., DISCONNECT was not received, so will is still non-null).
+    /// Called from CloseConnection(). Mirrors Go <c>mqttHandleWill()</c>.
+    /// </summary>
+    public static void DeliverWill(ClientConnection c)
+    {
+        MqttWill? will;
+        lock (c)
+        {
+            if (c.Mqtt == null) return;
+            will = c.Mqtt.Will;
+            c.Mqtt.Will = null; // Prevent duplicate delivery.
+        }
+
+        if (will == null)
+            return;
+
+        // Deliver the will message via internal NATS routing.
+        var payload = will.Msg ?? [];
+        c.ProcessInboundClientMsg(payload);
+    }
+
     // =========================================================================
     // PUBLISH parsing + processing
     // =========================================================================

dotnet/tests/ZB.MOM.NatsNet.Server.Tests/Mqtt/MqttConnectTests.cs

@@ -383,4 +383,68 @@ public sealed class MqttConnectTests
         // Will should be cleared.
         c.Mqtt.Will.ShouldBeNull();
     }
+
+    // =========================================================================
+    // Will message delivery tests
+    // =========================================================================
+
+    [Fact]
+    public void DeliverWill_AbnormalClose_ShouldDeliverAndClearWill()
+    {
+        var c = CreateMqttClient();
+
+        // CONNECT with a will.
+        var connectBuf = BuildConnectPacket(
+            willTopic: "test/will",
+            willMessage: Encoding.UTF8.GetBytes("goodbye"));
+
+        var err = MqttParser.Parse(c, connectBuf, connectBuf.Length);
+        err.ShouldBeNull();
+        c.Mqtt!.Will.ShouldNotBeNull();
+
+        // Simulate abnormal close (not DISCONNECT) — will should be delivered.
+        MqttPacketHandlers.DeliverWill(c);
+
+        // Will should be cleared after delivery.
+        c.Mqtt.Will.ShouldBeNull();
+    }
+
+    [Fact]
+    public void DeliverWill_AfterDisconnect_ShouldNotDeliver()
+    {
+        var c = CreateMqttClient();
+
+        // CONNECT with a will.
+        var connectBuf = BuildConnectPacket(
+            willTopic: "test/will",
+            willMessage: Encoding.UTF8.GetBytes("goodbye"));
+
+        var err = MqttParser.Parse(c, connectBuf, connectBuf.Length);
+        err.ShouldBeNull();
+
+        // Clean DISCONNECT — clears the will.
+        var disconnectBuf = new byte[] { MqttPacket.Disconnect, 0x00 };
+        err = MqttParser.Parse(c, disconnectBuf, disconnectBuf.Length);
+        err.ShouldBeNull();
+
+        // Will was already cleared by DISCONNECT, so DeliverWill should be a no-op.
+        MqttPacketHandlers.DeliverWill(c);
+        c.Mqtt!.Will.ShouldBeNull();
+    }
+
+    [Fact]
+    public void DeliverWill_NoWill_ShouldBeNoOp()
+    {
+        var c = CreateMqttClient();
+
+        // CONNECT without a will.
+        var connectBuf = BuildConnectPacket();
+        var err = MqttParser.Parse(c, connectBuf, connectBuf.Length);
+        err.ShouldBeNull();
+        c.Mqtt!.Will.ShouldBeNull();
+
+        // DeliverWill with no will configured — should be a no-op.
+        MqttPacketHandlers.DeliverWill(c);
+        c.Mqtt.Will.ShouldBeNull();
+    }
 }