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/PdfSignOptionsDto.cs

@@ -2,15 +2,53 @@ using EnvelopeGenerator.Application.Common.Enums;
 
 namespace EnvelopeGenerator.Application.Common.Dto;
 
+/// <summary>
+/// 
+/// </summary>
 public sealed record PdfSignOptionsDto
 {
-    public string? UUID { get; set; }
-    public string? Name { get; set; }
-    public string? Reason { get; set; }
-    public string? Location { get; set; }
-    public string? Email { get; set; }
-    public string? Phone { get; set; }
-    public string? ApplicationName { get; set; }
-    public PdfSignatureCertificationLevel? CertificationLevel { get; set; }
-    public PdfVisualSignatureOptionsDto? VisualSignature { get; set; }
+    /// <summary>
+    /// 
+    /// </summary>
+    public string UUID { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Name { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Reason { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Location { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Email { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string Phone { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public string ApplicationName { get; set; } = null!;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public PdfSignatureCertificationLevel CertificationLevel { get; set; }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    public PdfVisualSignatureOptionsDto VisualSignature { get; set; } = null!;
 }

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,