Add test for ReaderFrom hidden by nopWriteCloser

When using WithStdout, the writer is wrapped in nopWriteCloser,
which hides the ReaderFrom interface of the underlying writer.
This prevents io.CopyBuffer from dispatching to ReadFrom for
potential zero-copy paths (e.g., when the destination is a
network connection or has a custom ReadFrom implementation).

This test currently FAILS, demonstrating the problem.

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

Author: znull

pipe/iocopier_test.go

@@ -2,8 +2,10 @@ package pipe
 
 import (
 	"bytes"
+	"io"
 	"os"
 	"runtime"
+	"sync/atomic"
 	"testing"
 )
 
@@ -73,3 +75,47 @@ func TestIOCopierPoolBufferUsed(t *testing.T) {
 			allocBytes, maxBytes)
 	}
 }
+
+// readFromWriter is a test writer that implements io.ReaderFrom and
+// records whether ReadFrom was called.
+type readFromWriter struct {
+	bytes.Buffer
+	readFromCalled atomic.Bool
+}
+
+func (w *readFromWriter) ReadFrom(r io.Reader) (int64, error) {
+	w.readFromCalled.Store(true)
+	return w.Buffer.ReadFrom(r)
+}
+
+func (w *readFromWriter) Close() error { return nil }
+
+// TestIOCopierUsesReadFrom verifies that ioCopier dispatches to
+// ReaderFrom when the destination writer supports it, even when
+// wrapped in nopWriteCloser (as happens with WithStdout).
+func TestIOCopierUsesReadFrom(t *testing.T) {
+	const payload = "hello readfrom\n"
+
+	pr, pw, err := os.Pipe()
+	if err != nil {
+		t.Fatal(err)
+	}
+	go func() {
+		pw.Write([]byte(payload))
+		pw.Close()
+	}()
+
+	w := &readFromWriter{}
+	c := newIOCopier(nopWriteCloser{w})
+	c.Start(nil, Env{}, pr)
+	c.Wait()
+
+	if w.Buffer.String() != payload {
+		t.Fatalf("unexpected output: %q", w.Buffer.String())
+	}
+
+	if !w.readFromCalled.Load() {
+		t.Error("ioCopier did not call ReadFrom on destination; " +
+			"nopWriteCloser may be hiding the ReaderFrom interface")
+	}
+}