test(template): M2.8 review nits — stale-binding comment + stale-ID & inert-check tests (#23)

Add code comments in ValidateConnectionBindingCompleteness explaining
that the unbound-attribute branch also covers the silently-dropped
stale-binding case (cross-reference FlatteningService.ApplyConnectionBindings),
and that the `continue` skips the exists-at-site check for unbound attrs.

Add two new tests:
- FlatteningPipelineConnectionBindingTests: stale DataConnectionId (999)
  not present in site connections → flattener drops it silently →
  validator reports ConnectionBinding Error, IsValid false.
- ValidationServiceTests: enforce:true + siteConnectionNames:null on a
  properly-bound attribute → no ConnectionBinding error (exists-at-site
  check stays inert when site set is not supplied).

tests/ZB.MOM.WW.ScadaBridge.DeploymentManager.Tests/FlatteningPipelineConnectionBindingTests.cs

@@ -96,4 +96,27 @@ public class FlatteningPipelineConnectionBindingTests
         Assert.DoesNotContain(result.Value.Validation.Errors,
             e => e.Category == ValidationCategory.ConnectionBinding);
     }
+
+    [Fact]
+    public async Task FlattenAndValidate_BindingToStaleDeletedConnection_ReportsBindingError()
+    {
+        // M2.8 (#23): FlatteningService.ApplyConnectionBindings silently drops a
+        // binding whose DataConnectionId doesn't resolve to any loaded site
+        // DataConnection (stale / deleted connection). The flattener leaves
+        // BoundDataConnectionId == null, so the validator treats the attribute as
+        // unbound and gates the deployment with a ConnectionBinding Error.
+        //
+        // Arrange: the instance binding points at id 999, but the site only has
+        // the connection with id=ConnectionId (7). The flattener can't resolve 999
+        // and drops the binding silently; the validator then flags it.
+        const int StaleConnectionId = 999;
+        Arrange(boundConnectionId: StaleConnectionId);
+
+        var result = await _sut.FlattenAndValidateAsync(InstanceId);
+
+        Assert.True(result.IsSuccess);
+        Assert.False(result.Value.Validation.IsValid);
+        Assert.Contains(result.Value.Validation.Errors,
+            e => e.Category == ValidationCategory.ConnectionBinding);
+    }
 }