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/ValidatePdf/Queries/ValidatePdfQuery.cs

@@ -0,0 +1,28 @@
+using DocumentService.Application.Common.DTOs;
+using DocumentService.Application.Common.Interfaces;
+using MediatR;
+
+namespace DocumentService.Application.ValidatePdf.Queries;
+
+/// <summary>
+/// Query for PDF validation (Stream-based)
+/// </summary>
+public record ValidatePdfQuery : IRequest<PdfValidationResult>
+{
+    /// <summary>
+    /// PDF as stream (caller is responsible for disposal)
+    /// </summary>
+    public required Stream PdfStream { get; init; }
+}
+
+/// <summary>
+/// Handler for ValidatePdfQuery
+/// </summary>
+public class ValidatePdfQueryHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<ValidatePdfQuery, PdfValidationResult>
+{
+    public async Task<PdfValidationResult> Handle(ValidatePdfQuery request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.ValidateAsync(request.PdfStream);
+    }
+}