fix(esg): park buffered calls that fail deterministically with ArgumentException (path template/verb) instead of retrying to exhaustion (plan R2-06 T3)

src/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway/ExternalSystemClient.cs

@@ -166,7 +166,11 @@ public class ExternalSystemClient : IExternalSystemClient
     /// Delivers a buffered ExternalSystem call during a store-and-forward
     /// retry sweep. Returns true on success, false on permanent failure (the message
     /// is parked); throws <see cref="TransientExternalSystemException"/> on a
-    /// transient failure so the engine retries.
+    /// transient failure so the engine retries. Deterministic-poison failures against
+    /// the CURRENT method definition — malformed JSON, or an
+    /// <see cref="ArgumentException"/> from an unresolvable <c>{param}</c> path
+    /// template or an unsupported HTTP verb — return false (park immediately) rather
+    /// than retry, because re-running the same stored payload can never succeed.
     /// </summary>
     /// <param name="message">The buffered message to deliver.</param>
     /// <param name="cancellationToken">Cancellation token.</param>
@@ -221,6 +225,21 @@ public class ExternalSystemClient : IExternalSystemClient
             _logger.LogError(ex, "Buffered call to '{System}' failed permanently; parking.", payload.SystemName);
             return false;
         }
+        catch (ArgumentException ex)
+        {
+            // N2: same poison-message shape as the JsonException catch above. BuildUrl's
+            // {param} path-template substitution (placeholder with no matching/non-null
+            // parameter) and ValidateHttpMethod both throw ArgumentException
+            // deterministically for the SAME stored payload — the method definition
+            // changed underneath the buffered call, and retrying can never succeed.
+            // Return false so the S&F engine parks the message instead of counting the
+            // throw as transient and burning MaxRetries.
+            _logger.LogError(
+                ex,
+                "Buffered call to '{System}'/'{Method}' fails deterministically against the current method definition; parking.",
+                payload.SystemName, payload.MethodName);
+            return false;
+        }
         // TransientExternalSystemException propagates — the S&F engine retries.
     }