feat: Add MergePdfsCommand with merged Command/Handler/Validator

- Add MergePdfsCommand.cs (Vertical Slice pattern)
- Command: IRequest<byte[]> with PdfStreams + PageRanges properties
- Handler: Calls IPdfProcessor.MergePdfsAsync, uses primary constructor
- Validator: Validates minimum 2 PDFs, page ranges count matches PDF count
- All 3 classes in single file (Command/Handler/Validator merged)

DocumentOperator.Application/MergePdfs/MergePdfsCommand.cs

@@ -0,0 +1,53 @@
+using DocumentOperator.Application.Common.Interfaces;
+using FluentValidation;
+using MediatR;
+
+namespace DocumentOperator.Application.MergePdfs;
+
+/// <summary>
+/// Command to merge multiple PDF documents into a single PDF.
+/// </summary>
+public record MergePdfsCommand : IRequest<byte[]>
+{
+    /// <summary>
+    /// PDF streams to merge. Minimum 2 required. Each must be at Position=0.
+    /// </summary>
+    public required IReadOnlyList<Stream> PdfStreams { get; init; }
+
+    /// <summary>
+    /// Optional page ranges per PDF (null = all pages). 
+    /// Format: "1-3,5" means pages 1, 2, 3, and 5.
+    /// If provided, array length must match PdfStreams length.
+    /// </summary>
+    public IReadOnlyList<string?>? PageRanges { get; init; }
+}
+
+/// <summary>
+/// Handler for MergePdfsCommand.
+/// Delegates PDF merge operation to infrastructure layer.
+/// </summary>
+public class MergePdfsCommandHandler(IPdfProcessor pdfProcessor) 
+    : IRequestHandler<MergePdfsCommand, byte[]>
+{
+    public async Task<byte[]> Handle(MergePdfsCommand request, CancellationToken cancellationToken)
+    {
+        return await pdfProcessor.MergePdfsAsync(request.PdfStreams, request.PageRanges);
+    }
+}
+
+/// <summary>
+/// Validator for MergePdfsCommand.
+/// </summary>
+public class MergePdfsCommandValidator : AbstractValidator<MergePdfsCommand>
+{
+    public MergePdfsCommandValidator()
+    {
+        RuleFor(x => x.PdfStreams)
+            .NotNull()
+            .WithMessage("PDF streams are required");
+
+        RuleFor(x => x.PdfStreams)
+            .Must(streams => streams != null && streams.Count >= 2)
+            .WithMessage("At least 2 PDF files are required for merging");
+    }
+}