fix(external-system-gateway): resolve ExternalSystemGateway-012,013,014 — failure logging, connection-limit wiring, test coverage; ExternalSystemGateway-011 flagged

src/ScadaLink.ExternalSystemGateway/ExternalSystemClient.cs

@@ -272,10 +272,21 @@ public class ExternalSystemClient : IExternalSystemClient
 
             if (ErrorClassifier.IsTransient(response.StatusCode))
             {
+                // Transient failures are normal operation (handled by retry / S&F) —
+                // record at debug level only so the event log is not noisy.
+                _logger.LogDebug(
+                    "Transient HTTP {StatusCode} from external system {System} calling {Method}.",
+                    (int)response.StatusCode, system.Name, method.Name);
                 throw ErrorClassifier.AsTransient(
                     $"HTTP {(int)response.StatusCode} from {system.Name}: {errorBody}");
             }
 
+            // The design requires permanent failures to be visible in Site Event
+            // Logging — emit a warning so the gateway is not silent on a permanent
+            // failure (ExternalSystemGateway-012).
+            _logger.LogWarning(
+                "Permanent HTTP {StatusCode} from external system {System} calling {Method}: {Error}",
+                (int)response.StatusCode, system.Name, method.Name, errorBody);
             throw new PermanentExternalSystemException(
                 $"HTTP {(int)response.StatusCode} from {system.Name}: {errorBody}",
                 (int)response.StatusCode);