refactor(adminui): omit error key on success cert-audit rows + assert OccurredAtUtc (review)

src/Server/ZB.MOM.WW.OtOpcUa.AdminUI/Audit/CertAuditEvents.cs

@@ -28,18 +28,17 @@ public static class CertAuditEvents
     /// <see cref="AuditOutcome.Success" />); otherwise <see langword="false" /> (Outcome
     /// <see cref="AuditOutcome.Failure" />).</param>
     /// <param name="error">On failure, the error text carried in the details payload; ignored on
-    /// success (the details payload serializes <see langword="null" /> for the error field).</param>
+    /// success (the <c>error</c> field is omitted from the details payload entirely).</param>
     /// <returns>A fully populated <see cref="AuditEvent" /> with a fresh <see cref="AuditEvent.EventId" />
     /// and <see cref="AuditEvent.OccurredAtUtc" /> set to now (UTC).</returns>
     public static AuditEvent Build(
         string action, string store, string thumbprint, string actor, bool success, string? error)
     {
-        var detailsJson = JsonSerializer.Serialize(new
-        {
-            store,
-            thumbprint,
-            error = success ? null : error,
-        });
+        // On success the error field is omitted entirely (not serialized as null) so the common
+        // success-path audit row carries no dead "error" key.
+        var detailsJson = JsonSerializer.Serialize(success
+            ? (object)new { store, thumbprint }
+            : new { store, thumbprint, error });
 
         return new AuditEvent
         {