feat: Add CheckPdfAttachments feature - Application layer

- Add CheckPdfAttachmentsQuery with dual input support (byte[] + Base64)
- Add CheckPdfAttachmentsQueryHandler with IPdfProcessor integration
- Add CheckPdfAttachmentsQueryValidator with FluentValidation rules
- Add AttachmentCheckResult DTO for API response
- Handler converts byte[] → MemoryStream for IPdfProcessor.CheckAttachmentsAsync()
- AutoMapper maps AttachmentInfo → AttachmentCheckResult

DocumentOperator.Application/Common/DTOs/AttachmentCheckResult.cs

@@ -0,0 +1,43 @@
+namespace DocumentOperator.Application.Common.DTOs;
+
+/// <summary>
+/// DTO for attachment check result returned to API layer
+/// </summary>
+public record AttachmentCheckResult
+{
+    /// <summary>
+    /// Indicates whether the PDF contains any attachments
+    /// </summary>
+    public bool HasAttachments { get; init; }
+
+    /// <summary>
+    /// Total number of attachments in the PDF
+    /// </summary>
+    public int AttachmentCount { get; init; }
+
+    /// <summary>
+    /// List of attachment metadata (file details)
+    /// </summary>
+    public List<AttachmentDto> Attachments { get; init; } = new();
+}
+
+/// <summary>
+/// DTO for individual attachment metadata
+/// </summary>
+public record AttachmentDto
+{
+    /// <summary>
+    /// Attachment file name (e.g., "invoice.xml", "document.pdf")
+    /// </summary>
+    public string FileName { get; init; } = string.Empty;
+
+    /// <summary>
+    /// MIME type of the attachment (e.g., "text/xml", "application/pdf")
+    /// </summary>
+    public string MimeType { get; init; } = string.Empty;
+
+    /// <summary>
+    /// Attachment file size in bytes
+    /// </summary>
+    public long Size { get; init; }
+}