feat: deliver retained messages on MQTT SUBSCRIBE (Gap 6.5)

Add DeliverRetainedOnSubscribe to MqttRetainedStore, which iterates
GetMatchingRetained and fires a callback with (topic, payload, qos,
retain=true) for each match. Add 11 unit tests covering exact-match,
'+' single-level, '#' multi-level, no-match, cross-level rejection,
callback arguments, return count, and empty-store behaviour.

src/NATS.Server/Mqtt/MqttRetainedStore.cs

@@ -89,6 +89,20 @@ public sealed class MqttRetainedStore
         return results;
     }
 
+    /// <summary>
+    /// Delivers all retained messages matching the given topic filter to the provided callback.
+    /// The callback receives (topic, payload, qos, retain=true) for each matching message.
+    /// Returns the number of messages delivered.
+    /// Go reference: server/mqtt.go mqttGetRetainedMessages / mqttHandleRetainedMsg ~line 1650.
+    /// </summary>
+    public int DeliverRetainedOnSubscribe(string topicFilter, Action<string, byte[], byte, bool> deliver)
+    {
+        var matches = GetMatchingRetained(topicFilter);
+        foreach (var msg in matches)
+            deliver(msg.Topic, msg.Payload.ToArray(), 0, true);
+        return matches.Count;
+    }
+
     /// <summary>
     /// Sets (or clears) the retained message and persists to backing store.
     /// Go reference: server/mqtt.go mqttHandleRetainedMsg with JetStream.