Enhanced `PdfExtractionResult` with a new `ZugferdGuidelineId` property to store the ZUGFeRD Guideline-ID extracted from XMP metadata. Updated `Upload.cshtml` to display this information in the UI if available.
Implemented ZUGFeRD Guideline-ID extraction in `PdfAttachmentExtractorService` using a new helper method `ExtractGuidelineId`, which parses XMP metadata for known prefixes. Added logging for extracted Guideline-IDs and improved error handling with a `try-catch` block for metadata processing.
Fixed `FileAttachments` handling in `PdfAttachmentExtractorService` by converting to `List<T>` for proper `Count` property usage.
Added `PdfALevel` and `PdfAWarning` properties to the `PdfExtractionResult` class to store the PDF/A compliance level and indicate if a warning should be displayed for non-compliance.
Updated `Upload.cshtml` to display the PDF/A compliance level and conditionally show a warning message if the document is not PDF/A compliant.
Enhanced `PdfAttachmentExtractorService` to determine the PDF/A compliance level using `PdfDocumentProcessor`, map it to a string representation, and log the compliance level. Added logic to set `PdfAWarning` for non-compliant documents.
