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/ConvertFromPdfA/ConvertFromPdfACommand.cs

@@ -0,0 +1,41 @@
+using DocumentService.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentService.Application.ConvertFromPdfA;
+
+/// <summary>
+/// Command to convert a PDF/A document to a standard PDF (removes PDF/A restrictions)
+/// </summary>
+public record ConvertFromPdfACommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF/A document stream. Must be positioned at the beginning (Position = 0).
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+}
+
+/// <summary>
+/// Handler for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ConvertFromPdfACommand, byte[]>
+{
+    public async Task<byte[]> Handle(ConvertFromPdfACommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ConvertFromPdfAAsync(request.PdfStream);
+    }
+}
+
+/// <summary>
+/// Validator for ConvertFromPdfACommand
+/// </summary>
+public class ConvertFromPdfACommandValidator : AbstractValidator<ConvertFromPdfACommand>
+{
+    public ConvertFromPdfACommandValidator()
+    {
+        RuleFor(x => x.PdfStream)
+            .NotNull()
+            .WithMessage("PDF stream is required");
+    }
+}