Refactor: Rename DocumentOperator to DocumentService

Updated project and solution files to rename and restructure
`DocumentOperator` to `DocumentService`. Modified project
references in `DocumentService.API.csproj`, `DocumentService.Application.csproj`,
and `DocumentService.Infrastructure.csproj` to reflect the new
naming convention. Updated `DocumentService.sln` to remove
old project references, add new ones, and adjust configuration
and nested project mappings accordingly. This change aligns
the project structure with the new naming convention.

DocumentService.Application/ConvertToPdfA/ConvertToPdfACommand.cs

@@ -0,0 +1,59 @@
+using DocumentService.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentService.Application.ConvertToPdfA;
+
+/// <summary>
+/// Command to convert a standard PDF to PDF/A format
+/// </summary>
+public record ConvertToPdfACommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+
+    /// <summary>
+    /// Target PDF/A level (e.g., "PDF/A-1b", "PDF/A-2b", "PDF/A-3b")
+    /// </summary>
+    public required string PdfALevel { get; init; }
+}
+
+/// <summary>
+/// Handler for ConvertToPdfACommand
+/// </summary>
+public class ConvertToPdfACommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ConvertToPdfACommand, byte[]>
+{
+    public async Task<byte[]> Handle(ConvertToPdfACommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ConvertToPdfAAsync(request.PdfStream, request.PdfALevel);
+    }
+}
+
+/// <summary>
+/// Validator for ConvertToPdfACommand
+/// </summary>
+public class ConvertToPdfACommandValidator : AbstractValidator<ConvertToPdfACommand>
+{
+    private static readonly string[] ValidPdfALevels = 
+    {
+        "PDF/A-1b", "PDF/A-1a",
+        "PDF/A-2b", "PDF/A-2u", "PDF/A-2a",
+        "PDF/A-3b", "PDF/A-3u", "PDF/A-3a"
+    };
+
+    public ConvertToPdfACommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+
+        RuleFor(x => x.PdfALevel)
+            .NotEmpty()
+            .WithMessage("PDF/A level is required")
+            .Must(level => ValidPdfALevels.Contains(level, StringComparer.OrdinalIgnoreCase))
+            .WithMessage($"Invalid PDF/A level. Valid values: {string.Join(", ", ValidPdfALevels)}");
+    }
+}