test(mqtt): pin status-code half of republish suppression; note Sparkplug bypass

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

@@ -755,6 +755,10 @@ public sealed class MqttSubscriptionManager
         // sink write) per message. The cache is still freshened — a read must serve the current
         // timestamp — and only the notification is skipped. "Cached but not raised" is already a valid
         // state on this path (see Raise: an authored-but-unsubscribed tag does exactly that).
+        //
+        // Sparkplug bypasses this entirely. SparkplugIngestor.cs updates Values and raises directly
+        // (its own Values.Update()/Raise() calls, not through this Deliver method), so Sparkplug B
+        // ingestion gets no unchanged-value suppression — deliberately out of scope here.
         var prior = Values.Read(def.Name);
         var unchanged = prior.StatusCode == snapshot.StatusCode && !ValuesAreDifferent(prior.Value, snapshot.Value);
 

tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.Mqtt.Tests/MqttSubscriptionManagerTests.cs

@@ -1032,6 +1032,13 @@ public sealed class MqttSubscriptionManagerTests
     ///     <c>DegradeTopic</c>: a Good value followed by a refused subscription must reach the
     ///     subscriber as Bad, or the client keeps a stale Good forever.
     /// </summary>
+    /// <remarks>
+    ///     This pins the <c>DegradeTopic</c> path specifically — <c>DegradeTopic</c>'s own
+    ///     <c>Raise</c> call was already unconditional before the suppression change and is untouched
+    ///     by it. It does NOT exercise the <c>prior.StatusCode == snapshot.StatusCode</c> comparison
+    ///     added inside <see cref="MqttSubscriptionManager.Deliver"/>; see
+    ///     <see cref="Different_malformed_payloads_with_the_same_null_value_both_raise"/> for that.
+    /// </remarks>
     [Fact]
     public async Task A_status_transition_still_raises()
     {
@@ -1057,6 +1064,39 @@ public sealed class MqttSubscriptionManagerTests
         mgr.Values.Read(OvenTempPath).StatusCode.ShouldBe(BadCommunicationError);
     }
 
+    /// <summary>
+    ///     Pins the status-code half of <c>Deliver</c>'s suppression comparison
+    ///     (<c>prior.StatusCode == snapshot.StatusCode</c>) through the LIVE
+    ///     <see cref="MqttSubscriptionManager.HandleMessage"/> path, which
+    ///     <see cref="A_status_transition_still_raises"/> does not reach (that test goes through the
+    ///     unconditional <c>DegradeTopic</c> path instead). Two different malformed payloads on the
+    ///     SAME topic each decode to a <c>null</c> value but a DIFFERENT Bad status code — a decode
+    ///     failure (<c>BadDecodingError</c>) followed by a value that decodes but fails type coercion
+    ///     (<c>BadTypeMismatch</c>). A value-only comparison (ignoring status) would see
+    ///     <c>null == null</c> on both transitions and wrongly suppress the second.
+    /// </summary>
+    [Fact]
+    public async Task Different_malformed_payloads_with_the_same_null_value_both_raise()
+    {
+        var mgr = NewManager();
+        mgr.Register([Tag(OvenTempPath, $$"""{"topic":"{{OvenTopic}}","payloadFormat":"Json","jsonPath":"$.value","dataType":"Float64"}""")]);
+        await mgr.SubscribeAsync([OvenTempPath], TimeSpan.FromSeconds(1), TestContext.Current.CancellationToken);
+
+        var raised = new List<DataValueSnapshot>();
+        mgr.OnDataChange += (_, e) => raised.Add(e.Snapshot);
+
+        // Decode failure: not valid JSON at all.
+        mgr.HandleMessage(OvenTopic, "{not json"u8.ToArray(), retained: false);
+        // Decodes fine, but the selected value does not fit the declared Float64 type.
+        mgr.HandleMessage(OvenTopic, """{"value":"not-a-number"}"""u8.ToArray(), retained: false);
+
+        raised.Count.ShouldBe(2);
+        raised[0].StatusCode.ShouldBe(BadDecodingError);
+        raised[0].Value.ShouldBeNull();
+        raised[1].StatusCode.ShouldBe(BadTypeMismatch);
+        raised[1].Value.ShouldBeNull();
+    }
+
     /// <summary>Records every subscribe call so dedupe / re-subscribe can be asserted without a broker.</summary>
     private sealed class FakeTransport : IMqttSubscribeTransport
     {