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/PdfAttachmentRequests.cs

@@ -0,0 +1,46 @@
+namespace DocumentService.Application.Common.DTOs.Requests;
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF attachment check
+/// </summary>
+public record CheckPdfAttachmentsRequest : PdfBase64RequestBase;
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF attachment extraction
+/// </summary>
+public record ExtractPdfAttachmentsRequest : PdfBase64RequestBase;
+
+/// <summary>
+/// Request DTO for Base64-encoded PDF with attachments to add
+/// </summary>
+public record AddAttachmentsRequest : PdfBase64RequestBase
+{
+    /// <summary>
+    /// List of attachments to embed
+    /// </summary>
+    public required List<AttachmentRequestDto> Attachments { get; init; }
+}
+
+/// <summary>
+/// DTO for a single attachment file in a request
+/// </summary>
+public record AttachmentRequestDto
+{
+    /// <summary>
+    /// File name (e.g., "invoice.xml", "document.pdf")
+    /// </summary>
+    /// <example>factur-x.xml</example>
+    public required string FileName { get; init; }
+
+    /// <summary>
+    /// File content encoded as Base64 string
+    /// </summary>
+    /// <example>PD94bWwgdmVyc2lvbj0iMS4wIj8+...</example>
+    public required string Base64Content { get; init; }
+
+    /// <summary>
+    /// MIME type (optional, e.g., "application/xml")
+    /// </summary>
+    /// <example>application/xml</example>
+    public string? MimeType { get; init; }
+}