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/CheckPdfAttachments/Queries/CheckPdfAttachmentsQueryValidator.cs

@@ -0,0 +1,44 @@
+using FluentValidation;
+
+namespace DocumentOperator.Application.CheckPdfAttachments.Queries;
+
+/// <summary>
+/// Validator for CheckPdfAttachmentsQuery
+/// Ensures exactly one input type (PdfBytes OR Base64Pdf) is provided
+/// </summary>
+public class CheckPdfAttachmentsQueryValidator : AbstractValidator<CheckPdfAttachmentsQuery>
+{
+    public CheckPdfAttachmentsQueryValidator()
+    {
+        // Rule 1: Exactly ONE input must be provided (XOR logic)
+        RuleFor(x => x)
+            .Must(x => (x.PdfBytes != null && x.PdfBytes.Length > 0) ^
+                       (!string.IsNullOrWhiteSpace(x.Base64Pdf)))
+            .WithMessage("Either PdfBytes or Base64Pdf must be provided, but not both");
+
+        // Rule 2: Base64 format validation (if provided)
+        RuleFor(x => x.Base64Pdf)
+            .Must(BeValidBase64)
+            .When(x => !string.IsNullOrWhiteSpace(x.Base64Pdf))
+            .WithMessage("Base64Pdf must be a valid Base64 string");
+    }
+
+    /// <summary>
+    /// Validates if a string is a valid Base64 format
+    /// </summary>
+    private bool BeValidBase64(string? base64)
+    {
+        if (string.IsNullOrWhiteSpace(base64))
+            return true; // Skip validation if null/empty (handled by Rule 1)
+
+        try
+        {
+            Convert.FromBase64String(base64);
+            return true;
+        }
+        catch (FormatException)
+        {
+            return false;
+        }
+    }
+}