fix(dashboard): keep the '-' placeholder on failed /browse reads instead of '[redacted]'

src/ZB.MOM.WW.MxGateway.Server/Dashboard/DashboardLiveDataService.cs

@@ -53,9 +53,12 @@ public sealed class DashboardLiveDataService : IDashboardLiveDataService, IAsync
     /// <summary>
     ///     <c>MxGateway:Dashboard:ShowTagValues</c>. False (the default)
     ///     substitutes <see cref="DashboardTagValue.RedactedValueText"/> for every
-    ///     value this service hands the Browse panel; quality, data type, source
-    ///     timestamp, and any error still describe the real read, so the panel
-    ///     remains a diagnostic surface without being a value-disclosure one.
+    ///     successfully read value this service hands the Browse panel; quality,
+    ///     data type, source timestamp, and any error still describe the real
+    ///     read, so the panel remains a diagnostic surface without being a
+    ///     value-disclosure one. Failed reads keep their existing "-" placeholder:
+    ///     there was no value to suppress, so claiming one was withheld would
+    ///     misreport the failure.
     /// </summary>
     private readonly bool _showTagValues;
 
@@ -124,9 +127,17 @@ public sealed class DashboardLiveDataService : IDashboardLiveDataService, IAsync
             // The only place the /browse live-value gate is evaluated: the page
             // renders whatever ValueText it is handed, so a second check in the
             // view could only ever disagree with this one.
+            //
+            // A failed read is left alone: it has no value to suppress, and its
+            // ValueText is already the "-" placeholder. Substituting "[redacted]"
+            // there would tell the operator a value was withheld when the read
+            // never produced one — the Error column says why it failed, and the
+            // two must not contradict each other.
             DashboardTagValue[] values = results
                 .Select(DashboardTagValue.FromBulkReadResult)
-                .Select(value => _showTagValues ? value : value with { ValueText = DashboardTagValue.RedactedValueText })
+                .Select(value => _showTagValues || !value.Ok
+                    ? value
+                    : value with { ValueText = DashboardTagValue.RedactedValueText })
                 .ToArray();
             return new DashboardLiveReadResult(values, null, session.SessionId, session.WorkerProcessId);
         }