feat(batch47): implement MQTT full runtime — JSA bridge, sessions, account manager, protocol handlers

- MqttJsa.cs: full JetStream API bridge (22 features, Sub-batch A 2269-2290)
  - Async request/response helpers, consumer/stream CRUD, msg store/load/delete
  - Send queue via Channel<MqttJsPubMsg>, ConcurrentDictionary reply tracking

- MqttAccountSessionManager.cs: per-account MQTT session manager (26 features, Sub-batch C 2292-2322)
  - Session add/remove/lock/unlock, flapper tracking with cleanup timer
  - Retained message in-memory cache with TTL eviction (ConcurrentDictionary)
  - JSA reply dispatch, retained msg processing, session persist detection
  - Subscription creation, retained message subject matching (SubscriptionIndex)
  - createOrRestoreSession async (JetStream load + fallback to new session)
  - processSubs builds NATS subscriptions with retained message delivery
  - Stream migration stubs (transferUniqueSessStreamsToMuxed, transferRetained...)

- MqttTypes.cs: add Client and Seq to MqttSession; ExpiresFromCache to MqttRetainedMsg
  - Remove stubs for MqttJsa and MqttAccountSessionManager

- MqttHelpers.cs: standalone helpers (Sub-batch F 2264-2268)
  - IsMqttReservedSubscription, DecodeRetainedMessage, GeneratePubPerms,
    CheckPubRetainedPerms, TopicFilterContainsWildcard, TopicToNatsSubject

- ClientConnection.Mqtt.cs: Mqtt property stub on ClientConnection (Sub-batch E entry)

- Subscription.cs: add internal Mqtt (MqttSub?) and InternalCallback fields

All 2660 unit tests pass, 0 failures.

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

@@ -0,0 +1,41 @@
+// Copyright 2020-2026 The NATS Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// Adapted from server/mqtt.go in the NATS server Go source.
+
+using ZB.MOM.NatsNet.Server.Mqtt;
+
+namespace ZB.MOM.NatsNet.Server;
+
+// ============================================================================
+// ClientConnection — MQTT partial (Sub-batch E, features 2257-2384)
+// ============================================================================
+
+public sealed partial class ClientConnection
+{
+    // ------------------------------------------------------------------
+    // MQTT state field
+    // ------------------------------------------------------------------
+
+    /// <summary>
+    /// Per-connection MQTT handler, non-null only for MQTT client connections.
+    /// Mirrors Go <c>client.mqtt *mqtt</c> field in server/mqtt.go.
+    /// </summary>
+    internal MqttHandler? Mqtt { get; private set; }
+
+    /// <summary>
+    /// Attaches an MQTT handler to this connection.
+    /// Called during CONNECT processing.
+    /// </summary>
+    internal void InitMqtt(MqttHandler handler) => Mqtt = handler;
+}