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/Common/Behaviors/ValidationBehavior.cs

@@ -0,0 +1,54 @@
+using FluentValidation;
+using MediatR;
+
+namespace DocumentService.Application.Common.Behaviors;
+
+/// <summary>
+/// MediatR Pipeline Behavior that validates requests using FluentValidation
+/// Executes BEFORE the Handler
+/// </summary>
+public class ValidationBehavior<TRequest, TResponse> : IPipelineBehavior<TRequest, TResponse>
+    where TRequest : IRequest<TResponse>
+{
+    private readonly IEnumerable<IValidator<TRequest>> _validators;
+
+    public ValidationBehavior(IEnumerable<IValidator<TRequest>> validators)
+    {
+        _validators = validators;
+    }
+
+    public async Task<TResponse> Handle(
+        TRequest request,
+        RequestHandlerDelegate<TResponse> next,
+        CancellationToken cancellationToken)
+    {
+        // Wenn keine Validators registriert sind, direkt weiter zum Handler
+        if (!_validators.Any())
+        {
+            return await next();
+        }
+
+        // Validation Context erstellen
+        var context = new ValidationContext<TRequest>(request);
+
+        // Alle Validators parallel ausführen
+        var validationResults = await Task.WhenAll(
+            _validators.Select(v => v.ValidateAsync(context, cancellationToken))
+        );
+
+        // Fehler sammeln
+        var failures = validationResults
+            .Where(r => !r.IsValid)
+            .SelectMany(r => r.Errors)
+            .ToList();
+
+        // Bei Fehlern: ValidationException werfen (wird von Exception Middleware gefangen)
+        if (failures.Any())
+        {
+            throw new ValidationException(failures);
+        }
+
+        // Validation erfolgreich ? weiter zum Handler
+        return await next();
+    }
+}