fix(external-system-gateway): resolve ExternalSystemGateway-015..017 — treat MaxRetries=0 as unset, scope HTTP connection cap to gateway clients, no bare trailing '?'

src/ScadaLink.ExternalSystemGateway/ExternalSystemClient.cs

@@ -114,15 +114,20 @@ public class ExternalSystemClient : IExternalSystemClient
             // attempt above; letting EnqueueAsync re-invoke the handler would
             // dispatch the same request a second time.
             //
-            // The per-system retry settings are passed through verbatim — a
-            // configured MaxRetries of 0 means "never retry" and must NOT be
-            // collapsed to the S&F default (ExternalSystemGateway-004).
+            // ExternalSystemGateway-015: the entity's MaxRetries is a non-nullable
+            // int whose default is 0, and the Store-and-Forward engine interprets a
+            // stored MaxRetries of 0 as "no limit" (retry forever) — see
+            // StoreAndForwardMessage.MaxRetries ("0 = no limit") and the retry-sweep
+            // guard `MaxRetries > 0 && ...`. Passing 0 verbatim would therefore turn
+            // every unconfigured cached call into an unbounded retry loop. A 0 is
+            // treated as "unset" and passed as null so the bounded S&F default
+            // applies; the RetryDelay default of TimeSpan.Zero is likewise unset.
             await _storeAndForward.EnqueueAsync(
                 StoreAndForwardCategory.ExternalSystem,
                 systemName,
                 payload,
                 originInstanceName,
-                system.MaxRetries,
+                system.MaxRetries > 0 ? system.MaxRetries : null,
                 system.RetryDelay > TimeSpan.Zero ? system.RetryDelay : null,
                 attemptImmediateDelivery: false);
 
@@ -329,7 +334,15 @@ public class ExternalSystemClient : IExternalSystemClient
             var queryString = string.Join("&",
                 parameters.Where(p => p.Value != null)
                     .Select(p => $"{Uri.EscapeDataString(p.Key)}={Uri.EscapeDataString(p.Value?.ToString() ?? "")}"));
-            url += "?" + queryString;
+
+            // Only append "?" when the effective query string is non-empty — a method
+            // whose parameter values are all null produces no query string, and the
+            // URL must then be identical to the no-parameters case rather than ending
+            // in a bare "?" (ExternalSystemGateway-017).
+            if (queryString.Length > 0)
+            {
+                url += "?" + queryString;
+            }
         }
 
         return url;