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).

src/ZB.MOM.WW.ScadaBridge.TemplateEngine/Validation/ValidationService.cs

@@ -580,6 +580,17 @@ public class ValidationService
                 // Unbound data-sourced attribute. At deploy time this gates the
                 // deployment; at design time the binding is set later, so it is
                 // only advisory.
+                //
+                // NOTE: this branch fires for TWO distinct cases that are
+                // indistinguishable post-flattening:
+                //   1. The user genuinely never set a binding.
+                //   2. The user set a binding, but FlatteningService.ApplyConnectionBindings
+                //      silently dropped it because the stored DataConnectionId no longer
+                //      resolves to any loaded site DataConnection (i.e. the connection was
+                //      deleted after the binding was created). In that case the flattener
+                //      leaves BoundDataConnectionId == null, and the attribute falls into
+                //      this same "unbound → Error" path.
+                // The error message covers both cases; no behavioral change is needed.
                 if (enforce)
                 {
                     errors.Add(ValidationEntry.Error(ValidationCategory.ConnectionBinding,
@@ -592,6 +603,7 @@ public class ValidationService
                         $"Attribute '{attr.CanonicalName}' has a data source reference but no connection binding.",
                         attr.CanonicalName));
                 }
+                // Skip the "exists at site" check below — it only applies to bound attributes.
                 continue;
             }