mcp: don't close stdout when shutting down the stdio transport (#562)

We need to close stdin when the stdio transport is shut down, to unblock
its read loop. However, we don't technically need to close stdout.

Fixes #548

mcp/transport.go

@@ -90,9 +90,16 @@ type StdioTransport struct{}
 
 // Connect implements the [Transport] interface.
 func (*StdioTransport) Connect(context.Context) (Connection, error) {
-	return newIOConn(rwc{os.Stdin, os.Stdout}), nil
+	return newIOConn(rwc{os.Stdin, nopCloserWriter{os.Stdout}}), nil
 }
 
+// nopCloserWriter is an io.WriteCloser with a trivial Close method.
+type nopCloserWriter struct {
+	io.Writer
+}
+
+func (nopCloserWriter) Close() error { return nil }
+
 // An IOTransport is a [Transport] that communicates over separate
 // io.ReadCloser and io.WriteCloser using newline-delimited JSON.
 type IOTransport struct {
@@ -107,6 +114,9 @@ func (t *IOTransport) Connect(context.Context) (Connection, error) {
 
 // An InMemoryTransport is a [Transport] that communicates over an in-memory
 // network connection, using newline-delimited JSON.
+//
+// InMemoryTransports should be constructed using [NewInMemoryTransports],
+// which returns two transports connected to each other.
 type InMemoryTransport struct {
 	rwc io.ReadWriteCloser
 }