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/gotenberg/mocks.go

@@ -35,9 +35,12 @@ func (mod *ValidatorMock) Validate() error {
 }
 
 // PdfEngineMock is a mock for the [PdfEngine] interface.
+//
+//nolint:dupl
 type PdfEngineMock struct {
 	MergeMock         func(ctx context.Context, logger *zap.Logger, inputPaths []string, outputPath string) error
 	SplitMock         func(ctx context.Context, logger *zap.Logger, mode SplitMode, inputPath, outputDirPath string) ([]string, error)
+	FlattenMock       func(ctx context.Context, logger *zap.Logger, inputPath string) error
 	ConvertMock       func(ctx context.Context, logger *zap.Logger, formats PdfFormats, inputPath, outputPath string) error
 	ReadMetadataMock  func(ctx context.Context, logger *zap.Logger, inputPath string) (map[string]interface{}, error)
 	WriteMetadataMock func(ctx context.Context, logger *zap.Logger, metadata map[string]interface{}, inputPath string) error
@@ -51,6 +54,10 @@ func (engine *PdfEngineMock) Split(ctx context.Context, logger *zap.Logger, mode
 	return engine.SplitMock(ctx, logger, mode, inputPath, outputDirPath)
 }
 
+func (engine *PdfEngineMock) Flatten(ctx context.Context, logger *zap.Logger, inputPath string) error {
+	return engine.FlattenMock(ctx, logger, inputPath)
+}
+
 func (engine *PdfEngineMock) Convert(ctx context.Context, logger *zap.Logger, formats PdfFormats, inputPath, outputPath string) error {
 	return engine.ConvertMock(ctx, logger, formats, inputPath, outputPath)
 }