Add ZUGFeRD Guideline-ID extraction and display

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.

DXApp.TemplateKitProject/Pages/Invoices/Upload.cshtml

@@ -45,6 +45,13 @@
         @* PDF/A-Konformitätsstufe anzeigen *@
         <div class="alert @(Model.Result.PdfAWarning ? "alert-warning" : "alert-info") mt-2">
             <strong>PDF/A-Konformität:</strong> @Model.Result.PdfALevel
+            @if (!string.IsNullOrEmpty(Model.Result.ZugferdGuidelineId))
+            {
+                <br />
+    
+                <strong>Guideline-ID:</strong>
+                <code>@Model.Result.ZugferdGuidelineId</code>
+            }
             @if (Model.Result.PdfAWarning)
             {
                 <span> – ⚠️ ZUGFeRD-Rechnungen müssen PDF/A-3b sein.</span>