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/DocStatusBehavior.cs

@@ -0,0 +1,49 @@
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.DocStatus.Commands;
+using EnvelopeGenerator.Application.Signature.Commands;
+using EnvelopeGenerator.Domain.Constants;
+using MediatR;
+using System.Text.Json;
+
+namespace EnvelopeGenerator.Application.Signature.Behaviors;
+
+/// <summary>
+/// Pipeline behavior that creates document status.
+/// Executes second in the signing process.
+/// </summary>
+public class DocStatusBehavior : IPipelineBehavior<SignCommand, Unit>
+{
+    private const string BlankAnnotationJson = "{}";
+
+    private readonly ISender _sender;
+
+    /// <summary>
+    /// 
+    /// </summary>
+    /// <param name="sender"></param>
+    public DocStatusBehavior(ISender sender)
+    {
+        _sender = sender;
+    }
+
+    /// <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)
+    {
+        await _sender.Send(new CreateDocStatusCommand()
+        {
+            EnvelopeId = request.EnvelopeReceiver.EnvelopeId,
+            ReceiverId = request.EnvelopeReceiver.ReceiverId,
+            Value = request.PsPdfKitAnnotation is PsPdfKitAnnotation annot
+                        ? JsonSerializer.Serialize(annot.Instant, Format.Json.ForAnnotations)
+                        : BlankAnnotationJson
+        }, cancellationToken);
+
+        return await next();
+    }
+}