merge: MQTT browser+contracts review findings resolved (Driver.Mqtt.Browser-002..-005, Driver.Mqtt.Contracts-004..-007)

src/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt/MqttSubscriptionManager.cs

@@ -65,6 +65,14 @@ public sealed class MqttSubscriptionManager
     /// </summary>
     public const int DefaultMaxPayloadBytes = 1024 * 1024;
 
+    /// <summary>
+    ///     Subscription QoS applied when neither the tag nor the driver's <c>Plain</c> section authored
+    ///     one. Mirrors <see cref="MqttPlainOptions.DefaultQos"/>' own default — duplicated rather than
+    ///     referenced because that value is an instance property, and this is the fallback for the case
+    ///     where the whole section is absent.
+    /// </summary>
+    public const int DefaultQos = 1;
+
     // OPC UA status codes (values verified against Opc.Ua.StatusCodes, not recalled).
     private const uint StatusGood = 0x00000000u;
 
@@ -137,7 +145,13 @@ public sealed class MqttSubscriptionManager
 
         MaxPayloadBytes = maxPayloadBytes > 0 ? maxPayloadBytes : DefaultMaxPayloadBytes;
         _mode = options.Mode;
-        _defaultQos = options.Plain?.DefaultQos ?? 1;
+        // Clamped here for the same reason MaxPayloadBytes is: this constructor is the single
+        // enforcement point for the driver options the manager captures, and MQTT defines exactly three
+        // QoS levels — an out-of-range value would otherwise ride into BuildFilters and out onto the
+        // SUBSCRIBE verbatim. Clamp rather than fall back to the default, matching what
+        // MqttDriverFormModel.Normalize already persists, so the authoring surface and the driver cannot
+        // reach different answers for the same authored number.
+        _defaultQos = Math.Clamp(options.Plain?.DefaultQos ?? DefaultQos, 0, 2);
         _driverId = driverId;
         _transport = transport;
         _logger = logger;