Add IPdfProcessor interface and update ROADMAP.md

The `IPdfProcessor` interface was added to the `Application/Common/Interfaces/` directory. It includes the `ValidateAsync` method for validating PDFs and extracting metadata, with proper XML documentation and dependency on domain value objects.

Updated `ROADMAP.md` to mark Step 3.1 as completed, detailing the creation of the `IPdfProcessor` interface and its implementation status.

Removed the `<Folder Include="Common\Interfaces\" />` entry from `DocumentOperator.Application.csproj` to reflect the transition from a placeholder folder structure to actual implementation.

DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs

@@ -0,0 +1,16 @@
+using DocumentOperator.Domain.Models.ValueObjects;
+
+namespace DocumentOperator.Application.Common.Interfaces;
+
+public interface IPdfProcessor
+{
+    /// <summary>
+    /// Validates a PDF and extracts metadata.
+    /// </summary>
+    /// <param name="pdfBytes">PDF content as byte array</param>
+    /// <returns>PDF metadata (page count, size, version, attachments)</returns>
+    /// <exception cref="Domain.Common.Exceptions.PdfProcessingException">
+    /// Thrown when PDF is corrupted or cannot be processed
+    /// </exception>
+    Task<PdfMetadata> ValidateAsync(byte[] pdfBytes);
+}

DocumentOperator.Application/DocumentOperator.Application.csproj

@@ -17,7 +17,6 @@
   </ItemGroup>
 
   <ItemGroup>
-    <Folder Include="Common\Interfaces\" />
     <Folder Include="Common\Behaviors\" />
     <Folder Include="Common\DTOs\" />
     <Folder Include="Common\Mappings\" />