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/DatabaseGateway.cs

@@ -96,15 +96,20 @@ public class DatabaseGateway : IDatabaseGateway
             Parameters = parameters
         });
 
-        // The per-connection 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 turn every
+        // unconfigured cached write 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.CachedDbWrite,
             connectionName,
             payload,
             originInstanceName,
-            definition.MaxRetries,
+            definition.MaxRetries > 0 ? definition.MaxRetries : null,
             definition.RetryDelay > TimeSpan.Zero ? definition.RetryDelay : null);
     }