Fix Go stderr capture: wait for reader goroutine before accessing buffer

Use StderrPipe with explicit ReadAll in goroutine, wait for EOF signal
before accessing buffer. This ensures all stderr data is captured even
when the process exits quickly.

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

Author: SteveSandersonMS

go/client.go

@@ -34,6 +34,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"net"
 	"os"
 	"os/exec"
@@ -48,6 +49,11 @@ import (
 	"github.com/github/copilot-sdk/go/rpc"
 )
 
+// readAll reads all data from r until EOF or error, returning the data read.
+func readAll(r io.Reader) ([]byte, error) {
+	return io.ReadAll(r)
+}
+
 // Client manages the connection to the Copilot CLI server and provides session management.
 //
 // The Client can either spawn a CLI server process or connect to an existing server.
@@ -1092,22 +1098,35 @@ func (c *Client) startCLIServer(ctx context.Context) error {
 			return fmt.Errorf("failed to create stdout pipe: %w", err)
 		}
 
-		// Capture stderr directly to buffer (not via pipe, which gets closed on Wait())
-		c.process.Stderr = &c.stderrBuf
+		stderr, err := c.process.StderrPipe()
+		if err != nil {
+			return fmt.Errorf("failed to create stderr pipe: %w", err)
+		}
 
 		if err := c.process.Start(); err != nil {
 			return fmt.Errorf("failed to start CLI server: %w", err)
 		}
 
+		// Read stderr in background - reads until EOF (process exit) then signals completion
+		stderrDone := make(chan struct{})
+		go func() {
+			// ReadAll reads until EOF, which happens when process terminates
+			data, _ := readAll(stderr)
+			c.stderrBuf.Write(data)
+			close(stderrDone)
+		}()
+
 		// Monitor process exit to signal pending requests
 		c.processDone = make(chan struct{})
 		go func() {
-			err := c.process.Wait()
+			waitErr := c.process.Wait()
+			// Wait for stderr reader to finish before accessing buffer
+			<-stderrDone
 			stderrOutput := strings.TrimSpace(c.stderrBuf.String())
 			if stderrOutput != "" {
-				c.processError = fmt.Errorf("CLI process exited: %v\nstderr: %s", err, stderrOutput)
-			} else if err != nil {
-				c.processError = fmt.Errorf("CLI process exited: %v", err)
+				c.processError = fmt.Errorf("CLI process exited: %v\nstderr: %s", waitErr, stderrOutput)
+			} else if waitErr != nil {
+				c.processError = fmt.Errorf("CLI process exited: %v", waitErr)
 			} else {
 				c.processError = fmt.Errorf("CLI process exited unexpectedly")
 			}