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.
Enhanced `PdfSignOptionsDto` with a `VisualSignature` property and introduced the `PdfVisualSignatureOptionsDto` class for configuring visual signature options. Updated `PdfDigitalSignatureService` to handle visual signatures, including page number resolution, signature bounds calculation, and appearance customization.
Refactored `PdfSignatureController` to support visual signatures by extending `SignPdfForm` with `SignatureImageFile` and `VSignature` properties. Improved validation for page numbers and signature dimensions. Streamlined `Sign` and `IsSigned` endpoints for better clarity and reduced redundancy.
Added XML documentation for improved API clarity and performed general code cleanup to enhance maintainability.
