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/PdfBase64RequestBase.cs

@@ -0,0 +1,14 @@
+namespace DocumentService.Application.Common.DTOs.Requests;
+
+/// <summary>
+/// Base record for all single-PDF Base64 request types.
+/// Provides the common <see cref="Base64Pdf"/> property shared across endpoints.
+/// </summary>
+public abstract record PdfBase64RequestBase
+{
+    /// <summary>
+    /// PDF document encoded as Base64 string
+    /// </summary>
+    /// <example>JVBERi0xLjQKJeLjz9MKMSAwIG9iago8PC9UeXBlL0NhdGFsb2c...</example>
+    public required string Base64Pdf { get; init; }
+}