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/CheckPdfAttachmentsQuery.cs

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+using AutoMapper;
+using DocumentOperator.Application.Common.DTOs;
+using DocumentOperator.Application.Common.Interfaces;
+using MediatR;
+
+namespace DocumentOperator.Application.CheckPdfAttachments.Queries;
+
+/// <summary>
+/// Query for checking PDF attachments (supports both byte array and Base64 input)
+/// </summary>
+public record CheckPdfAttachmentsQuery : IRequest<AttachmentCheckResult>
+{
+    /// <summary>
+    /// PDF as byte array (direct upload via multipart/form-data)
+    /// </summary>
+    public byte[]? PdfBytes { get; init; }
+
+    /// <summary>
+    /// PDF as Base64 string (for API clients using application/json)
+    /// </summary>
+    public string? Base64Pdf { get; init; }
+}
+
+/// <summary>
+/// Handler for CheckPdfAttachmentsQuery
+/// Orchestrates PDF attachment checking using IPdfProcessor and AutoMapper
+/// </summary>
+public class CheckPdfAttachmentsQueryHandler(IPdfProcessor PdfProcessor, IMapper Mapper)
+    : IRequestHandler<CheckPdfAttachmentsQuery, AttachmentCheckResult>
+{
+    /// <summary>
+    /// Checks PDF attachments and returns detailed metadata
+    /// </summary>
+    public async Task<AttachmentCheckResult> Handle(CheckPdfAttachmentsQuery request, CancellationToken cancellationToken)
+    {
+        // Use byte[] if available, otherwise convert Base64
+        byte[] pdfBytes = request.PdfBytes ?? Convert.FromBase64String(request.Base64Pdf!);
+
+        // Convert to stream for IPdfProcessor
+        using var pdfStream = new MemoryStream(pdfBytes);
+
+        // Call DevExpress service (exceptions propagate naturally)
+        var attachmentInfo = await PdfProcessor.CheckAttachmentsAsync(pdfStream);
+
+        // Map DTO to response DTO using AutoMapper
+        return Mapper.Map<AttachmentCheckResult>(attachmentInfo);
+    }
+}

DocumentOperator.Application/CheckPdfAttachments/Queries/CheckPdfAttachmentsQueryValidator.cs

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+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;
+        }
+    }
+}

DocumentOperator.Application/Common/DTOs/AttachmentCheckResult.cs

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+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; }
+}