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/MqttKeepAliveTests.cs

@@ -11,6 +11,7 @@ public class MqttKeepAliveTests
     public async Task Invalid_mqtt_credentials_or_keepalive_timeout_close_session_with_protocol_error()
     {
         await using var listener = new MqttListener("127.0.0.1", 0);
+        listener.UseBinaryProtocol = false;
         using var cts = new CancellationTokenSource();
         await listener.StartAsync(cts.Token);