fix: Handle ObjectDisposedException when disposing linked CancellationTokenSource (#1714)

When the engine cancellation token is cancelled, the linked
CancellationTokenSource in PrintProgressExecutor may already be
disposed by the time DisposeAsync is called. This causes an
ObjectDisposedException when CancelAfter is called.

Wrap the CancelAfter call in a try-catch to gracefully handle
this race condition.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

Author: thomhurst

src/ModularPipelines/Engine/Executors/PrintProgressExecutor.cs

@@ -46,7 +46,14 @@ public async Task<IPrintProgressExecutor> InitializeAsync()
 
     public async ValueTask DisposeAsync()
     {
-        _printProgressCancellationTokenSource?.CancelAfter(ProgressPrinterGracePeriodMs);
+        try
+        {
+            _printProgressCancellationTokenSource?.CancelAfter(ProgressPrinterGracePeriodMs);
+        }
+        catch (ObjectDisposedException)
+        {
+            // Linked CancellationTokenSource may already be disposed if the engine token was cancelled
+        }
 
         await SafelyAwaitProgressPrinter().ConfigureAwait(false);