Eliminated a redundant `using DevExpress.Pdf;` statement to improve code readability and maintain cleaner imports. This change has no functional impact but enhances code clarity.
Replaced `DevExpress.Pdf.Core` with `DevExpress.Document.Processor` in `DocumentOperator.Infrastructure.csproj` to adopt a newer library for PDF processing. Removed the `Services\PdfProcessing\` folder reference.
Updated `DevExpressPdfProcessorTests` to reflect changes in the `ValidateAsync` method's behavior.
Introduced the `DevExpressPdfProcessor` class, implementing the `IPdfProcessor` interface. This class validates PDF documents and extracts metadata using the `DevExpress.Pdf` library. Added defensive input validation, metadata extraction, and exception handling for domain consistency.
