Rename SignCommand to SigningCommand across codebase

Updated all references to `SignCommand` to use the new `SigningCommand` name for consistency and clarity. This includes changes to class definitions, method signatures, and pipeline behaviors in the following files:

- `DependencyInjection.cs`: Updated pipeline behaviors to use `SigningCommand`.
- `AnnotationBehavior.cs`: Updated class definition and methods to use `SigningCommand`. Marked `SignCommand` as `[Obsolete]`.
- `DocStatusBehavior.cs`, `EnvelopeReceiverResolutionBehavior.cs`, `HistoryBehavior.cs`, `SendSignedMailBehavior.cs`: Updated class definitions and methods to use `SigningCommand`.
- `SendSignedMailBehavior.cs`: Updated `CreatePlaceHolders` method to accept `SigningCommand`.
- `SigningCommand.cs`: Renamed `SignCommand` record to `SigningCommand` and updated internal methods and properties. Renamed `SignCommandHandler` to `SigningCommandHandler`.

Marked `SignCommand` as `[Obsolete]` where applicable to indicate deprecation. These changes improve code readability and align the command name with its purpose in the signing process.

EnvelopeGenerator.Application/Signatures/Behaviors/AnnotationBehavior.cs

@@ -11,7 +11,7 @@ namespace EnvelopeGenerator.Application.Signatures.Behaviors;
 /// Executes first in the signing process.
 /// </summary>
 [Obsolete("This notification is deprecated. Use Signature.Commands.SignCommand instead.")]
-public class AnnotationBehavior : IPipelineBehavior<SignCommand, Unit>
+public class AnnotationBehavior : IPipelineBehavior<SigningCommand, Unit>
 {
     private readonly IRepository<ElementAnnotation> _repo;
 
@@ -31,7 +31,7 @@ public class AnnotationBehavior : IPipelineBehavior<SignCommand, Unit>
     /// <param name="next"></param>
     /// <param name="cancellationToken"></param>
     /// <returns></returns>
-    public async Task<Unit> Handle(SignCommand request, RequestHandlerDelegate<Unit> next, CancellationToken cancellationToken)
+    public async Task<Unit> Handle(SigningCommand request, RequestHandlerDelegate<Unit> next, CancellationToken cancellationToken)
     {
         if (request.PsPdfKitAnnotation is PsPdfKitAnnotation annot)
             await _repo.CreateAsync(annot.Structured, cancellationToken);