fix(esg): cancellation-aware ErrorClassifier.IsTransient overload — caller-cancelled work can never classify transient

Claude-Session: https://claude.ai/code/session_01MtdgwpEeCUn6cUA5f1LMPj

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

@@ -26,6 +26,10 @@ public static class ErrorClassifier
 
     /// <summary>
     /// Determines whether an exception represents a transient failure.
+    /// Prefer the cancellation-aware <see cref="IsTransient(Exception, CancellationToken)"/>
+    /// overload at call sites that hold the caller's token — this token-less overload
+    /// classifies every <see cref="OperationCanceledException"/> as transient, including
+    /// caller-requested cancellations that must never be buffered for S&amp;F retry.
     /// </summary>
     /// <param name="exception">The exception to classify.</param>
     /// <returns><see langword="true"/> for connection/timeout/cancellation exceptions; <see langword="false"/> otherwise.</returns>
@@ -37,6 +41,18 @@ public static class ErrorClassifier
             or OperationCanceledException;
     }
 
+    /// <summary>
+    /// Cancellation-aware overload: a caller-requested cancellation is NOT transient
+    /// (buffering caller-abandoned work into S&amp;F would be wrong). Prefer this overload
+    /// at call sites that hold the caller's token.
+    /// </summary>
+    public static bool IsTransient(Exception exception, CancellationToken cancellationToken)
+    {
+        if (exception is OperationCanceledException && cancellationToken.IsCancellationRequested)
+            return false;
+        return IsTransient(exception);
+    }
+
     /// <summary>
     /// Creates a TransientException for S&amp;F buffering.
     /// </summary>

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

@@ -340,7 +340,7 @@ public class ExternalSystemClient : IExternalSystemClient
             throw ErrorClassifier.AsTransient(
                 $"Timeout calling {system.Name} after {effectiveTimeout.TotalSeconds:0.##}s", ex);
         }
-        catch (Exception ex) when (ErrorClassifier.IsTransient(ex))
+        catch (Exception ex) when (ErrorClassifier.IsTransient(ex, cancellationToken))
         {
             throw ErrorClassifier.AsTransient($"Connection error to {system.Name}: {ex.Message}", ex);
         }

tests/ZB.MOM.WW.ScadaBridge.ExternalSystemGateway.Tests/ErrorClassifierTests.cs

@@ -64,4 +64,13 @@ public class ErrorClassifierTests
         Assert.IsType<TransientExternalSystemException>(ex);
         Assert.Equal("test message", ex.Message);
     }
+
+    [Fact]
+    public void IsTransient_CallerCancelledOce_IsNotTransient()
+    {
+        var cts = new CancellationTokenSource();
+        cts.Cancel();
+        Assert.False(ErrorClassifier.IsTransient(new OperationCanceledException(), cts.Token));
+        Assert.True(ErrorClassifier.IsTransient(new OperationCanceledException(), CancellationToken.None));
+    }
 }