Refactor DTOs and namespaces for maintainability

Consolidated DTOs under `DocumentService.Application.Common.DTOs.Requests`
to centralize and align them with the application layer. Introduced
`PdfBase64RequestBase` to encapsulate shared properties, reducing
redundancy across request DTOs. Updated controllers, clients, and tests
to use the new DTO structure. Removed redundant DTOs and cleaned up
unused namespaces and comments. Improved documentation and ensured
consistent naming conventions across the codebase.

DocumentService.Application/Common/DTOs/Requests/ZugferdRequests.cs

@@ -0,0 +1,11 @@
+namespace DocumentService.Application.Common.DTOs.Requests;
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF ZUGFeRD check
+/// </summary>
+public record HasZugferdRequest : PdfBase64RequestBase;
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF ZUGFeRD extraction
+/// </summary>
+public record ExtractZugferdRequest : PdfBase64RequestBase;