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

tests/ScadaLink.ExternalSystemGateway.Tests/DatabaseGatewayTests.cs

@@ -113,8 +113,13 @@ public class DatabaseGatewayTests
     }
 
     [Fact]
-    public async Task CachedWrite_ZeroMaxRetriesIsHonouredNotTreatedAsUnset()
+    public async Task CachedWrite_ZeroMaxRetriesIsTreatedAsUnsetNotRetryForever()
     {
+        // ExternalSystemGateway-015: a stored MaxRetries of 0 is interpreted by the
+        // Store-and-Forward retry sweep as "no limit" (retry forever). The entity's
+        // non-nullable int default is also 0, so the gateway must treat the
+        // connection's MaxRetries == 0 as "unset" and pass null — the bounded S&F
+        // default must apply, never 0.
         var conn = new DatabaseConnectionDefinition("testDb", "Server=localhost;Database=test")
         {
             Id = 1,
@@ -144,7 +149,9 @@ public class DatabaseGatewayTests
         await gateway.CachedWriteAsync("testDb", "INSERT INTO t VALUES (1)");
 
         var (maxRetries, _) = ReadBufferedRetrySettings(connStr);
-        Assert.Equal(0, maxRetries); // honoured — not the S&F default of 99
+        // Must be the bounded S&F default, never 0 — a stored 0 would mean retry-forever.
+        Assert.Equal(99, maxRetries);
+        Assert.NotEqual(0, maxRetries);
     }
 
     private static (int MaxRetries, long RetryIntervalMs) ReadBufferedRetrySettings(string connStr)