fix: harden custom QR login output

client_test.go

@@ -37,6 +37,26 @@ func TestWithQRWriterRejectsNil(t *testing.T) {
 	}
 }
 
+func TestWithQRWriterRejectsTypedNil(t *testing.T) {
+	var output *bytes.Buffer
+	_, err := NewClient(t.TempDir(), WithQRWriter(output))
+	if err == nil || err.Error() != "QR writer must not be nil" {
+		t.Fatalf("error = %v, want QR writer must not be nil", err)
+	}
+}
+
+func TestWithQRWriterAcceptsStructWriter(t *testing.T) {
+	if _, err := NewClient(t.TempDir(), WithQRWriter(structWriter{})); err != nil {
+		t.Fatal(err)
+	}
+}
+
+type structWriter struct{}
+
+func (structWriter) Write(data []byte) (int, error) {
+	return len(data), nil
+}
+
 func TestDefaultQRWriterIsStdout(t *testing.T) {
 	client, err := NewClient(t.TempDir())
 	if err != nil {