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

tests/ZB.MOM.WW.ScadaBridge.TemplateEngine.Tests/Flattening/DiffServiceTests.cs

@@ -286,6 +286,36 @@ public class DiffServiceTests
         Assert.Equal("OpcUa", diff[0].NewValue!.Protocol);
     }
 
+    [Fact]
+    public void ComputeConnectionsDiff_NullOldConfig_AllReportedAsAdded()
+    {
+        // First deploy: there is no prior flattened config at all (null), so
+        // every connection in the new config is Added. Exercises the public
+        // method's null-oldConfig tolerance explicitly (the ComputeDiff path
+        // covers it end-to-end, but the isolated API contract is asserted here).
+        var newConfig = new FlattenedConfiguration
+        {
+            InstanceUniqueName = "Instance1",
+            Connections = new Dictionary<string, ConnectionConfig>
+            {
+                ["plc1"] = new ConnectionConfig
+                {
+                    Protocol = "OpcUa",
+                    ConfigurationJson = "{\"endpoint\":\"opc.tcp://host\"}",
+                    FailoverRetryCount = 3,
+                }
+            }
+        };
+
+        var diff = _sut.ComputeConnectionsDiff(null, newConfig);
+
+        Assert.Single(diff);
+        Assert.Equal("plc1", diff[0].CanonicalName);
+        Assert.Equal(DiffChangeType.Added, diff[0].ChangeType);
+        Assert.Null(diff[0].OldValue);
+        Assert.Equal("OpcUa", diff[0].NewValue!.Protocol);
+    }
+
     [Fact]
     public void ComputeConnectionsDiff_BindingCleared_ReportedAsRemoved()
     {