feat: wire MQTT end-to-end through NATS SubList for cross-protocol messaging

- MqttListener accepts IMessageRouter + delegates for client ID allocation
  and account resolution (Phase 1-2)
- MqttConnection creates MqttNatsClientAdapter on CONNECT, registers with
  SubList for cross-protocol delivery (Phase 2)
- PUBLISH routes through ProcessMessage() when router available, falls back
  to MQTT-only fan-out for test compatibility (Phase 3)
- SUBSCRIBE creates real SubList entries via adapter, enabling NATS→MQTT
  delivery with topic↔subject translation (Phase 4)
- PUBREL now delivers stored QoS 2 messages before ack (Phase 5)
- ConnzHandler includes MQTT adapters in /connz output (Phase 6)
- MQTTnet E2E tests: MQTT pub/sub, MQTT→NATS, NATS→MQTT, QoS 1 (Phase 7)

src/NATS.Server/NatsServer.cs

@@ -122,6 +122,12 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
     /// </summary>
     public int? MqttListenerPort => _mqttListener?.Port;
 
+    /// <summary>
+    /// Returns all active MQTT client adapters for monitoring (/connz).
+    /// </summary>
+    public IEnumerable<Mqtt.MqttNatsClientAdapter> GetMqttAdapters()
+        => _mqttListener?.GetMqttAdapters() ?? [];
+
     public Account SystemAccount => _systemAccount;
     public string ServerNKey { get; }
     public InternalEventSystem? EventSystem => _eventSystem;
@@ -937,7 +943,10 @@ public sealed class NatsServer : IMessageRouter, ISubListAccess, IDisposable
                 _authService,
                 mqttOptions,
                 mqttStreamInit,
-                mqttConsumerMgr);
+                mqttConsumerMgr,
+                router: this);
+            _mqttListener.AllocateClientId = () => Interlocked.Increment(ref _nextClientId);
+            _mqttListener.ResolveAccount = name => GetOrCreateAccount(name ?? Auth.Account.GlobalAccountName);
             await _mqttListener.StartAsync(linked.Token);
         }
         if (_jetStreamService != null)