feat(pdfengines): add support for flattening annotations (#1105)

* initial changes

* Add tests

* Fix edge case when we need to regenerate appearances

* Fix comments

* Add missing comment

* Add missing comment

* Add missing comment

* Add flatten option to the merge route

* Add flatten option to the libreoffice convert route

* Add flatten option to the chromium convert route

* Revert "Add flatten option to the chromium convert route"

This reverts commit cdab8b4e6b.

* Ignore lint false positives

* Add missing tests

* Add flatten route tests

* Replace input instead of creating a new file

* create copy before flatten in tests

---------

Co-authored-by: Peter Chakalov <peter.chakalov@abraxa.com>

pkg/modules/pdfcpu/pdfcpu.go

@@ -107,6 +107,11 @@ func (engine *PdfCpu) Split(ctx context.Context, logger *zap.Logger, mode gotenb
 	return outputPaths, nil
 }
 
+// Flatten is not available in this implementation.
+func (engine *PdfCpu) Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error {
+	return fmt.Errorf("flatten PDF with pdfcpu: %w", gotenberg.ErrPdfEngineMethodNotSupported)
+}
+
 // Convert is not available in this implementation.
 func (engine *PdfCpu) Convert(ctx context.Context, logger *zap.Logger, formats gotenberg.PdfFormats, inputPath, outputPath string) error {
 	return fmt.Errorf("convert PDF to '%+v' with pdfcpu: %w", formats, gotenberg.ErrPdfEngineMethodNotSupported)

pkg/modules/pdfcpu/pdfcpu_test.go

@@ -239,6 +239,15 @@ func TestPdfCpu_Split(t *testing.T) {
 	}
 }
 
+func TestPdfCpu_Flatten(t *testing.T) {
+	mod := new(PdfCpu)
+	err := mod.Flatten(context.TODO(), zap.NewNop(), "")
+
+	if !errors.Is(err, gotenberg.ErrPdfEngineMethodNotSupported) {
+		t.Errorf("expected error %v, but got: %v", gotenberg.ErrPdfEngineMethodNotSupported, err)
+	}
+}
+
 func TestPdfCpu_Convert(t *testing.T) {
 	mod := new(PdfCpu)
 	err := mod.Convert(context.TODO(), zap.NewNop(), gotenberg.PdfFormats{}, "", "")