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/IPdfValidationClient.cs

@@ -13,7 +13,7 @@ public interface IPdfValidationClient
     /// <param name="pdfStream">PDF file stream</param>
     /// <param name="cancellationToken">Cancellation token</param>
     /// <returns>Validation result with metadata</returns>
-    Task<PdfValidationResult> ValidatePdfAsync(Stream pdfStream, CancellationToken cancellationToken = default);
+    Task<PdfValidationResult> ValidateAsync(Stream pdfStream, CancellationToken cancellationToken = default);
 
     /// <summary>
     /// Validates a PDF from byte array (Base64 JSON).
@@ -21,7 +21,7 @@ public interface IPdfValidationClient
     /// <param name="pdfBytes">PDF file as byte array</param>
     /// <param name="cancellationToken">Cancellation token</param>
     /// <returns>Validation result with metadata</returns>
-    Task<PdfValidationResult> ValidatePdfAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
+    Task<PdfValidationResult> ValidateAsync(byte[] pdfBytes, CancellationToken cancellationToken = default);
 
     /// <summary>
     /// Validates a PDF/A from stream (multipart upload).