Add test showing startCmdWithPipe returns a nil reader on pipe failure

NewCmdTask feeds the reader returned by its start func straight into a
bufio.Scanner, whose Scan panics on a nil reader with a nil pointer
dereference. startCmdWithPipe returns exactly that when the pipe cannot
be created.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkg/gui/tasks_adapter_test.go

@@ -0,0 +1,27 @@
+package gui
+
+import (
+	"bytes"
+	"os/exec"
+	"testing"
+
+	"github.com/jesseduffield/lazygit/pkg/utils"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestStartCmdWithPipeWhenPipeCannotBeCreated(t *testing.T) {
+	cmd := exec.Command("non-existent-command")
+	// Assigning stdout up front makes cmd.StdoutPipe fail. This happens in
+	// practice on the Unix pty fallback path: a failed pty start can leave
+	// the tty assigned to the command's stdout.
+	cmd.Stdout = &bytes.Buffer{}
+
+	_, r := startCmdWithPipe(cmd, utils.NewDummyLog())
+
+	// NewCmdTask's scanner panics on a nil reader, so startCmdWithPipe must
+	// not return one even when it can't create the pipe.
+	/* EXPECTED:
+	assert.NotNil(t, r)
+	ACTUAL: */
+	assert.Nil(t, r)
+}