Transfers: let cancel pressed as a transfer finishes do nothing rather than throw

src/DodoSSH.Client.Transfer/FileTransferQueue.cs

@@ -338,7 +338,16 @@ public sealed class FileTransferQueue : IAsyncDisposable
             }
         }
 
+        // Outside the gate, because cancelling runs the token's callbacks on this thread — but that leaves a
+        // moment in which the run finishes and disposes the source it was read from. A disposed source is a
+        // transfer that is already over, so there is nothing left to cancel and nothing to report.
+        try
+        {
             running?.Cancel();
+        }
+        catch (ObjectDisposedException)
+        {
+        }
 
         if (cancelled is not null)
         {