Fix StreamClientSessionTransport swallowing caller-triggered cancellation

When the caller's CancellationToken triggers an OperationCanceledException
during a pipe write, re-throw it instead of wrapping it in IOException.
This ensures callers get the expected OperationCanceledException when they
cancel their own token.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73

src/ModelContextProtocol.Core/Client/StreamClientSessionTransport.cs

@@ -85,6 +85,10 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
             await _serverInputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
             await _serverInputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
         }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw;
+        }
         catch (Exception ex)
         {
             LogTransportSendFailed(Name, id, ex);