Add pipeline behaviors for signing process

Introduced four new pipeline behaviors (`AnnotationBehavior`,
`DocStatusBehavior`, `HistoryBehavior`, and `SendSignedMailBehavior`)
to modularize the signing process. These behaviors handle annotation
persistence, document status creation, history recording, and signed
mail notifications, respectively.

- `AnnotationBehavior`: Saves annotations using `IRepository`.
- `DocStatusBehavior`: Creates document status via `ISender`.
- `HistoryBehavior`: Records signing history and validates receiver
  information.
- `SendSignedMailBehavior`: Sends signed mail notifications using
  email templates and dynamic placeholder replacement.

Added error handling for missing data in `HistoryBehavior` and
`SendSignedMailBehavior`. Updated the signing pipeline to execute
these behaviors sequentially, ensuring a structured and extensible
workflow.

EnvelopeGenerator.Application/Signature/Behaviors/AnnotationBehavior.cs

@@ -0,0 +1,40 @@
+using DigitalData.Core.Abstraction.Application.Repository;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Signature.Commands;
+using EnvelopeGenerator.Domain.Entities;
+using MediatR;
+
+namespace EnvelopeGenerator.Application.Signature.Behaviors;
+
+/// <summary>
+/// Pipeline behavior that saves annotations.
+/// Executes first in the signing process.
+/// </summary>
+public class AnnotationBehavior : IPipelineBehavior<SignCommand, Unit>
+{
+    private readonly IRepository<ElementAnnotation> _repo;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="repository"></param>
+    public AnnotationBehavior(IRepository<ElementAnnotation> repository)
+    {
+        _repo = repository;
+    }
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="next"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public async Task<Unit> Handle(SignCommand request, RequestHandlerDelegate<Unit> next, CancellationToken cancellationToken)
+    {
+        if (request.PsPdfKitAnnotation is PsPdfKitAnnotation annot)
+            await _repo.CreateAsync(annot.Structured, cancellationToken);
+
+        return await next();
+    }
+}