Refactor DTOs and simplify PdfDigitalSignatureService

Refactored `PdfSignOptionsDto` and `PdfVisualSignatureOptionsDto`:
- Converted both to immutable `record` types with non-nullable properties.
- Added XML documentation comments for properties and parameters.

Simplified `PdfDigitalSignatureService`:
- Updated `SignPdfAsync` to enforce non-nullable `PdfSignOptionsDto`.
- Removed redundant null checks and inlined `CreateSignatureBounds`.
- Simplified signature appearance and image data logic.
- Refactored `TryGetKeySize` to use `bool` with `out` parameter.

General improvements:
- Enhanced code readability and maintainability.
- Reduced nullable references and enforced stricter type safety.

EnvelopeGenerator.Application/Common/Dto/PdfVisualSignatureOptionsDto.cs

@@ -1,5 +1,15 @@
 namespace EnvelopeGenerator.Application.Common.Dto;
 
+/// <summary>
+/// 
+/// </summary>
+/// <param name="ImageBytes"></param>
+/// <param name="PageNumber"></param>
+/// <param name="Left"></param>
+/// <param name="Bottom"></param>
+/// <param name="Width"></param>
+/// <param name="Height"></param>
+/// <param name="ShowLabels"></param>
 public sealed record PdfVisualSignatureOptionsDto(
     byte[] ImageBytes,
     int PageNumber,