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

src/ScadaLink.ExternalSystemGateway/ErrorClassifier.cs

@@ -11,6 +11,10 @@ public static class ErrorClassifier
 {
     /// <summary>
     /// Determines whether an HTTP status code represents a transient failure.
+    /// Transient: HTTP 5xx, 408 (Request Timeout) and 429 (Too Many Requests).
+    /// Every other non-success status (the remaining 4xx) defaults to permanent —
+    /// a permanent failure is the safe default because retrying a 4xx is unlikely to
+    /// succeed and risks duplicate side effects.
     /// </summary>
     public static bool IsTransient(HttpStatusCode statusCode)
     {

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);

src/ScadaLink.ExternalSystemGateway/ServiceCollectionExtensions.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Options;
 using ScadaLink.Commons.Interfaces.Services;
 
 namespace ScadaLink.ExternalSystemGateway;
@@ -11,6 +12,22 @@ public static class ServiceCollectionExtensions
             .BindConfiguration("ScadaLink:ExternalSystemGateway");
 
         services.AddHttpClient();
+
+        // ExternalSystemGateway-013: wire MaxConcurrentConnectionsPerSystem into the
+        // primary handler of every per-system named client ("ExternalSystem_{name}"),
+        // so the option an operator configures actually bounds concurrent connections
+        // instead of being silently ignored. ConfigureHttpClientDefaults applies to
+        // the dynamically-named clients created by ExternalSystemClient.
+        services.ConfigureHttpClientDefaults(builder =>
+            builder.ConfigurePrimaryHttpMessageHandler(sp =>
+            {
+                var options = sp.GetRequiredService<IOptions<ExternalSystemGatewayOptions>>().Value;
+                return new SocketsHttpHandler
+                {
+                    MaxConnectionsPerServer = options.MaxConcurrentConnectionsPerSystem,
+                };
+            }));
+
         services.AddScoped<ExternalSystemClient>();
         services.AddScoped<IExternalSystemClient>(sp => sp.GetRequiredService<ExternalSystemClient>());
         services.AddScoped<DatabaseGateway>();