fix(dcl): treat OPC UA BadResourceUnavailable as transient Bad; Bad+null keeps last-known value (ScadaBridge#112)

OtOpcUa's out-of-process script runner publishes 0x80040000 with a null value.
- OpcUaDataConnection: logs entry/recovery once per tag, names the symbolic
  status in read errors; never a disconnect (not in the reconnect status set).
- InstanceActor: a Bad update with no value is a quality-only transition
  (keep value, quality Bad, stream publish, no script/alarm/waiter re-eval),
  matching the connection-drop rule. Bad with a value is unchanged.

Claude-Session: https://claude.ai/code/session_01GT67AEHd2Acv1YPwGfXEoV

src/ZB.MOM.WW.ScadaBridge.SiteRuntime/Actors/InstanceActor.cs

@@ -1223,6 +1223,23 @@ public class InstanceActor : ReceiveActor
                 traced = true;
             }
 
+            // Bad quality WITHOUT a value (ScadaBridge#112) is a quality-only transition: the
+            // source is saying "I cannot give you a value right now", not "the value is now
+            // null". OPC UA servers commonly omit the value on a Bad status — OtOpcUa always
+            // does, e.g. BadResourceUnavailable (0x80040000) from its out-of-process script
+            // runner — and the DCL's own tag-resolution failure pushes Bad+null too. Treat it
+            // exactly like a connection drop (HandleConnectionQualityChanged, DCL design
+            // "Connection drop detection"): keep the last-known value, flip quality to Bad,
+            // publish to the stream, and do NOT re-notify scripts/alarms or evaluate waiters —
+            // nulling the stored value would fire value-change scripts with a null and make a
+            // computed alarm re-evaluate on a value that never existed. A Bad update that DOES
+            // carry a value still takes the normal path below.
+            if (update.Quality == QualityCode.Bad && update.Value is null)
+            {
+                ApplyQualityOnlyBad(attrName, update.TagPath, update.Timestamp, traceSource, traced);
+                continue;
+            }
+
             // A List-typed attribute coerces the incoming OPC UA array
             // (a CLR array/IEnumerable from the SDK) into a typed List<T>. On an
             // element-type mismatch we set the attribute's quality to Bad, log a
@@ -1271,6 +1288,22 @@ public class InstanceActor : ReceiveActor
         }
     }
 
+    /// <summary>
+    /// Marks a data-sourced attribute Bad while keeping its last-known value (ScadaBridge#112):
+    /// quality + timestamp move, the stored value does not, and only the site stream / debug
+    /// view is told — mirroring <see cref="HandleConnectionQualityChanged"/>.
+    /// </summary>
+    private void ApplyQualityOnlyBad(
+        string attrName, string tagPath, DateTimeOffset timestamp, Guid? traceSource, bool traced)
+    {
+        _attributeQualities[attrName] = "Bad";
+        _attributeTimestamps[attrName] = timestamp;
+        _attributes.TryGetValue(attrName, out var currentValue);
+        _streamManager?.PublishAttributeValueChanged(new AttributeValueChanged(
+            _instanceUniqueName, tagPath, attrName, currentValue, "Bad", timestamp,
+            SourceExecutionId: traceSource, Trace: traced, SourceExecutionInferred: traced));
+    }
+
     /// <summary>Parses a <see cref="ResolvedAttribute.DataType"/> string, defaulting to
     /// <see cref="DataType.String"/> on an unrecognized value (never throws).</summary>
     private static DataType ParseDataType(string dataType) =>