Refactor: Standardize method names across clients

Renamed methods across multiple client classes for consistency,
shortening and aligning naming conventions (e.g., `CheckAttachmentsAsync` → `CheckAsync`, `ValidatePdfAsync` → `ValidateAsync`).

Updated corresponding interfaces, unit tests, and documentation
to reflect the new method names. Standardized method signatures
to support both `Stream` and `byte[]` overloads consistently.

Improved error handling in tests and updated examples in the
README for batch processing and validation scenarios. Enhanced
API endpoint overview for clarity.

DocumentService.Client/Interfaces/ISwissQrCodeClient.cs

@@ -14,7 +14,7 @@ public interface ISwissQrCodeClient
     /// <param name="raw">If true, returns raw QR text lines instead of parsed Bill object</param>
     /// <param name="cancellationToken">Cancellation token</param>
     /// <returns>Swiss QR Code extraction result</returns>
-    Task<SwissQrCodeExtractionResult> ExtractSwissQrCodeAsync(Stream pdfStream, bool raw = false, CancellationToken cancellationToken = default);
+    Task<SwissQrCodeExtractionResult> ExtractAsync(Stream pdfStream, bool raw = false, CancellationToken cancellationToken = default);
 
     /// <summary>
     /// Extracts Swiss QR Code from PDF (Base64).
@@ -23,6 +23,6 @@ public interface ISwissQrCodeClient
     /// <param name="raw">If true, returns raw QR text lines instead of parsed Bill object</param>
     /// <param name="cancellationToken">Cancellation token</param>
     /// <returns>Swiss QR Code extraction result</returns>
-    Task<SwissQrCodeExtractionResult> ExtractSwissQrCodeAsync(byte[] pdfBytes, bool raw = false, CancellationToken cancellationToken = default);
+    Task<SwissQrCodeExtractionResult> ExtractAsync(byte[] pdfBytes, bool raw = false, CancellationToken cancellationToken = default);
 }