feat: Add IPdfProcessor.MergePdfsAsync interface

- Add MergePdfsAsync method to IPdfProcessor interface
- Parameters: IReadOnlyList<Stream> pdfStreams, IReadOnlyList<string?>? pageRanges
- Returns: Task<byte[]> (merged PDF)
- Validates: Minimum 2 PDFs, stream Position = 0, page ranges count matches PDF count
- Supports optional page ranges (e.g., '1-3,5' or null for all pages)

DocumentOperator.Application/Common/Interfaces/IPdfProcessor.cs

@@ -58,4 +58,22 @@ public interface IPdfProcessor
     /// Thrown when PDF contains no attachments
     /// </exception>
     Task<byte[]> ExtractAttachmentsAsync(Stream pdfStream);
+
+    /// <summary>
+    /// Merges multiple PDF documents into a single PDF.
+    /// </summary>
+    /// <param name="pdfStreams">
+    /// PDF streams to merge (minimum 2 required). Each stream must be readable and positioned 
+    /// at the beginning (Position = 0). Caller is responsible for disposal.
+    /// </param>
+    /// <param name="pageRanges">
+    /// Optional page ranges per PDF (null = all pages). Format: "1-3,5" means pages 1, 2, 3, and 5.
+    /// If null or empty for a PDF, all pages are included. Array length must match pdfStreams length if provided.
+    /// </param>
+    /// <returns>Merged PDF as byte array</returns>
+    /// <exception cref="Domain.Common.Exceptions.BadRequestException">
+    /// Thrown when fewer than 2 PDFs provided, any stream is empty/invalid/not at Position=0, 
+    /// or page range format is invalid
+    /// </exception>
+    Task<byte[]> MergePdfsAsync(IReadOnlyList<Stream> pdfStreams, IReadOnlyList<string?>? pageRanges = null);
 }