Propagate caller-triggered cancellation in transport exception handling

Cherry-pick from halter73/transport-exceptions (fd2d71e, 19a1f2c):

- StreamClientSessionTransport.SendMessageAsync: re-throw
  OperationCanceledException when the caller's token triggers it,
  instead of wrapping in IOException.
- StreamServerTransport.SendMessageAsync: same pattern.
- SseClientSessionTransport.ConnectAsync: add OperationCanceledException
  guard and change InvalidOperationException to IOException for
  consistency (connection failure is I/O, not invalid state).

Co-authored-by: Stephen Halter <halter73@gmail.com>

Agent-Logs-Url: https://github.com/modelcontextprotocol/csharp-sdk/sessions/f1ba1333-443c-4239-8205-c32db49e9f22

Co-authored-by: halter73 <54385+halter73@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Client/SseClientSessionTransport.cs

@@ -58,11 +58,15 @@ public async Task ConnectAsync(CancellationToken cancellationToken = default)
 
             await _connectionEstablished.Task.WaitAsync(_options.ConnectionTimeout, cancellationToken).ConfigureAwait(false);
         }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw;
+        }
         catch (Exception ex)
         {
             LogTransportConnectFailed(Name, ex);
             await CloseAsync().ConfigureAwait(false);
-            throw new InvalidOperationException("Failed to connect transport", ex);
+            throw new IOException("Failed to connect transport.", ex);
         }
     }
 

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);

src/ModelContextProtocol.Core/Server/StreamServerTransport.cs

@@ -80,6 +80,10 @@ public override async Task SendMessageAsync(JsonRpcMessage message, Cancellation
             await _outputStream.WriteAsync(s_newlineBytes, cancellationToken).ConfigureAwait(false);
             await _outputStream.FlushAsync(cancellationToken).ConfigureAwait(false);
         }
+        catch (OperationCanceledException) when (cancellationToken.IsCancellationRequested)
+        {
+            throw;
+        }
         catch (Exception ex)
         {
             LogTransportSendFailed(Name, id, ex);