feat: implement full MQTT Go parity across 5 phases — binary protocol, auth/TLS, cross-protocol bridging, monitoring, and JetStream persistence

Phase 1: Binary MQTT 3.1.1 wire protocol with PipeReader-based parsing,
full packet type dispatch, and MQTT 3.1.1 compliance checks.

Phase 2: Auth pipeline routing MQTT CONNECT through AuthService,
TLS transport with SslStream wrapping, pinned cert validation.

Phase 3: IMessageRouter refactor (NatsClient → INatsClient),
MqttNatsClientAdapter for cross-protocol bridging, MqttTopicMapper
with full Go-parity topic/subject translation.

Phase 4: /connz mqtt_client field population, /varz actual MQTT port.

Phase 5: JetStream persistence — MqttStreamInitializer creates 5
internal streams, MqttConsumerManager for QoS 1/2 consumers,
subject-keyed session/retained lookups replacing linear scans.

All 503 MQTT tests and 1589 Core tests pass.

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

@@ -17,6 +17,7 @@ public class MqttWillMessageParityTests
     public async Task Subscriber_receives_message_on_abrupt_publisher_disconnect()
     {
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);
 
@@ -44,6 +45,7 @@ public class MqttWillMessageParityTests
     public async Task Qos1_will_message_is_delivered_to_subscriber()
     {
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);
 
@@ -72,6 +74,7 @@ public class MqttWillMessageParityTests
     public async Task Graceful_disconnect_does_not_deliver_extra_messages()
     {
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);
 
@@ -104,6 +107,7 @@ public class MqttWillMessageParityTests
     public async Task Will_message_at_various_qos_levels_reaches_subscriber(int qos, string payload)
     {
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);
 
@@ -205,6 +209,7 @@ public class MqttWillMessageParityTests
         _ = subQos;
 
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);