fix: resolve flaky EngineCancellationTokenTests by handling scheduler cancellation

When a module fails and the pipeline is cancelled, the scheduler throws
OperationCanceledException from WaitForNextSchedulingOpportunity. This
exception was not being caught, causing RegisterTerminatedResultsForCancelledModules
to be skipped. Dependent modules never got their results registered,
causing the test to intermittently fail.

The fix catches OperationCanceledException from the scheduler task when
it's caused by a module failure (firstException != null), allowing the
result registration to proceed correctly.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

src/ModularPipelines/Engine/ModuleExecutor.cs

@@ -129,7 +129,17 @@ private async Task<IEnumerable<IModule>> ExecuteWithSchedulerAsync(
             EnsureCancellation(cancellationTokenSource);
         }
 
-        await schedulerTask.ConfigureAwait(false);
+        try
+        {
+            await schedulerTask.ConfigureAwait(false);
+        }
+        catch (OperationCanceledException) when (firstException != null)
+        {
+            // Expected when cancellation was triggered due to module failure.
+            // The scheduler throws OperationCanceledException from WaitForNextSchedulingOpportunity
+            // when the cancellation token is cancelled.
+            _logger.LogDebug("Scheduler cancelled due to module failure");
+        }
 
         _logger.LogDebug("All modules completed");
 

test/ModularPipelines.UnitTests/Execution/EngineCancellationTokenTests.cs

@@ -67,9 +67,7 @@ public async Task When_Cancel_Engine_Token_With_DependsOn_Then_Modules_Cancel()
 
         await Assert.That(async () => await host.ExecutePipelineAsync()).ThrowsException();
 
-        // Allow time for all module results to be finalized after pipeline termination
-        await Task.Delay(TimeSpan.FromSeconds(1));
-
+        // Results should be registered before the exception is thrown, no delay needed
         var module1Result = resultRegistry.GetResult(typeof(Module1));
         // Module1 depends on BadModule which failed, so Module1 should be marked as PipelineTerminated
         await Assert.That(module1Result).IsNotNull();