- add `DefaultIndexOfAnnot` property to provide a fallback index for annotations
- add `GetAnnotationIndex(string name)` method to safely retrieve an annotation index, returning default if not found
- Updated BurnPdfCommandHandler constructor to accept IOptions<PDFBurnerParams> instead of raw PDFBurnerParams.
- Updated AddFormFieldValue to calculate coordinates based on PDFBurnerParams offsets and margins.
- Added helper methods for converting pixels to inches (ToInches, ToPointF).
- Removed unused NotImplemented methods in AddFormFieldValue overload.
- Improved code readability and maintainability for annotation burning logic.
