Add WorkflowsClient for PDF validation and QR extraction

Introduced a new `WorkflowsClient` to orchestrate multi-step
operations, combining `IPdfValidationClient` and
`ISwissQrCodeClient`. Added methods to validate PDFs and extract
Swiss QR Codes in a single call, with overloads for byte arrays,
streams, and file paths. Registered `WorkflowsClient` as a scoped
service in `ServiceCollectionExtensions`.

Updated `PdfValidationResult` with an `IsValid` property to
simplify PDF validity checks. Added `IWorkflowsClient` interface
to define the contract for the new client, with detailed XML
documentation for its methods.

DocumentService.Client/Extensions/ServiceCollectionExtensions.cs

@@ -58,6 +58,9 @@ public static class ServiceCollectionExtensions
         services.AddHttpClient<IPdfConversionClient, PdfConversionClient>(ConfigureClient)
             .ConfigurePrimaryHttpMessageHandler(configureHandler);
 
+        // WorkflowsClient composes existing clients – no HttpClient needed.
+        services.AddScoped<IWorkflowsClient, WorkflowsClient>();
+
         return services;
     }
 }