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/Clients/PdfValidationClient.cs

@@ -12,7 +12,7 @@ namespace DocumentService.Client.Clients;
 public class PdfValidationClient(HttpClient httpClient, ILogger<PdfValidationClient> logger) : BaseDocumentClient(httpClient, logger), IPdfValidationClient
 {
     /// <inheritdoc />
-    public async Task<PdfValidationResult> ValidatePdfAsync(Stream pdfStream, CancellationToken cancellationToken = default)
+    public async Task<PdfValidationResult> ValidateAsync(Stream pdfStream, CancellationToken cancellationToken = default)
     {
         var result = await SendMultipartAsync<PdfValidationResult>(
             "/api/pdf/validation/validate",
@@ -24,7 +24,7 @@ public class PdfValidationClient(HttpClient httpClient, ILogger<PdfValidationCli
     }
 
     /// <inheritdoc />
-    public async Task<PdfValidationResult> ValidatePdfAsync(byte[] pdfBytes, CancellationToken cancellationToken = default)
+    public async Task<PdfValidationResult> ValidateAsync(byte[] pdfBytes, CancellationToken cancellationToken = default)
     {
         var request = new ValidatePdfBase64Request
         {