fix: only throw PipelineFailedException on success path

When an exception occurs during pipeline execution (e.g. ModuleFailedException),
the original exception should be rethrown after OnEnd() completes. The previous
implementation incorrectly threw PipelineFailedException in the catch block,
replacing the original exception.

This fix adds a throwOnFailure parameter to OnEnd() to distinguish:
- Success path: throwOnFailure=true, may throw PipelineFailedException
- Error path: throwOnFailure=false, preserves original exception

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

Author: thomhurst

src/ModularPipelines/Engine/Executors/ExecutionOrchestrator.cs

@@ -119,17 +119,18 @@ private async Task<PipelineSummary> ExecuteInternal(CancellationToken cancellati
         {
             var result = await ExecutePipeline(runnableModules, organizedModules).ConfigureAwait(false);
 
-            return await OnEnd(organizedModules, stopWatch, start, result).ConfigureAwait(false);
+            return await OnEnd(organizedModules, stopWatch, start, result, throwOnFailure: true).ConfigureAwait(false);
         }
         catch
         {
-            await OnEnd(organizedModules, stopWatch, start, null).ConfigureAwait(false);
+            // Don't throw PipelineFailedException here - we want to rethrow the original exception
+            await OnEnd(organizedModules, stopWatch, start, null, throwOnFailure: false).ConfigureAwait(false);
             throw;
         }
     }
 
     private async Task<PipelineSummary> OnEnd(OrganizedModules organizedModules, Stopwatch stopWatch, DateTimeOffset start,
-        PipelineSummary? pipelineSummary)
+        PipelineSummary? pipelineSummary, bool throwOnFailure)
     {
         var end = DateTimeOffset.UtcNow;
         pipelineSummary ??= _pipelineSummaryFactory.Create(organizedModules.AllModules, stopWatch.Elapsed, start, end);
@@ -154,7 +155,8 @@ private async Task<PipelineSummary> OnEnd(OrganizedModules organizedModules, Sto
         }
 
         // Throw exception if pipeline failed - this ensures non-zero exit code in CI
-        if (pipelineSummary.Status == Status.Failed && _options.Value.ThrowOnPipelineFailure)
+        // Only throw when throwOnFailure is true (success path), not when handling an existing exception
+        if (throwOnFailure && pipelineSummary.Status == Status.Failed && _options.Value.ThrowOnPipelineFailure)
         {
             var failedModules = pipelineSummary.GetFailedModuleResults()
                 .Select(r => r.ModuleName)