fix(deploy): address M2.2 review nits — backup endpoint in diff summary + null-oldConfig test (#10)

- FormatConnection now includes BackupConfigurationJson so a backup-only change
  no longer renders identical Before/After cells (covers all 4 ConnectionsEqual fields)
- add ComputeConnectionsDiff(null, newConfig) first-deploy unit test

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/Topology.razor

@@ -892,13 +892,20 @@
     }
 
     // Compact summary of a connection's deployment-relevant fields for the diff
-    // table's Before/After cells: protocol, primary endpoint config, and the
-    // failover retry count. Mirrors the fields ConnectionsEqual compares.
+    // table's Before/After cells. Surfaces all four fields ConnectionsEqual
+    // compares — protocol, primary endpoint config, failover retry count, and
+    // the backup endpoint — so a backup-only change doesn't show identical
+    // Before/After cells. The backup segment is omitted when there is no backup.
     private static string FormatConnection(
         ZB.MOM.WW.ScadaBridge.Commons.Types.Flattening.ConnectionConfig c)
     {
         var endpoint = string.IsNullOrWhiteSpace(c.ConfigurationJson) ? "—" : c.ConfigurationJson;
-        return $"{c.Protocol} · {endpoint} · failover ×{c.FailoverRetryCount}";
+        var summary = $"{c.Protocol} · {endpoint} · failover ×{c.FailoverRetryCount}";
+        if (!string.IsNullOrWhiteSpace(c.BackupConfigurationJson))
+        {
+            summary += $" · backup {c.BackupConfigurationJson}";
+        }
+        return summary;
     }
 
     // Renders one change section (a heading plus a Bootstrap change-table) for a