Add Swiss QR Code extraction feature

Introduced DTOs for request and response to handle Swiss QR Code extraction from PDF documents. Implemented `ExtractSwissQrCodeHandler` to process the extraction using `ISwissQrCodeProcessor`. Added validation for the query with `ExtractSwissQrCodeValidator`. Developed unit tests to ensure correct behavior for successful and failure scenarios.

DocumentOperator.Application/Features/Documents/ExtractSwissQrCode/ExtractSwissQrCodeValidator.cs

@@ -0,0 +1,21 @@
+using FluentValidation;
+
+namespace DocumentOperator.Application.Features.Documents.ExtractSwissQrCode;
+
+/// <summary>
+/// Validates ExtractSwissQrCodeQuery before handler execution.
+/// Ensures references array and PDF content are provided.
+/// </summary>
+public sealed class ExtractSwissQrCodeValidator : AbstractValidator<ExtractSwissQrCodeQuery>
+{
+    public ExtractSwissQrCodeValidator()
+    {
+        RuleFor(x => x.References)
+            .NotNull()
+            .WithMessage("References array is required.");
+
+        RuleFor(x => x.PdfContent)
+            .NotNull()
+            .WithMessage("PDF content is required.");
+    }
+}