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/Commands/SigningCommand.cs

@@ -0,0 +1,52 @@
+using MediatR;
+using EnvelopeGenerator.Application.Common.Dto;
+using EnvelopeGenerator.Application.Common.Dto.EnvelopeReceiver;
+using EnvelopeGenerator.Application.Common.Query;
+
+namespace EnvelopeGenerator.Application.Signatures.Commands;
+
+/// <summary>
+/// Command to sign a document by a receiver.
+/// </summary>
+public record SigningCommand : EnvelopeReceiverQueryBase, IRequest
+{
+    private EnvelopeReceiverDto? _envelopeReceiver;
+
+    internal void SetEnvelopeReceiver(EnvelopeReceiverDto envelopeReceiver)
+    {
+        _envelopeReceiver = envelopeReceiver;
+    }
+
+    /// <summary>
+    /// The envelope receiver information.
+    /// </summary>
+    public EnvelopeReceiverDto EnvelopeReceiver
+    { 
+        get => _envelopeReceiver!;
+        init => _envelopeReceiver = value;
+    }
+
+    /// <summary>
+    /// The PSPDFKit annotation data.
+    /// </summary>
+    [Obsolete("This notification is deprecated. Use Signature.Commands.SignCommand instead.")]
+    public PsPdfKitAnnotation? PsPdfKitAnnotation { get; init; }
+}
+
+/// <summary>
+/// Handles the sign command. All work is done by pipeline behaviors.
+/// This handler is intentionally empty - behaviors handle all the processing.
+/// </summary>
+public class SignCommandHandler : IRequestHandler<SigningCommand>
+{
+    /// <summary>
+    /// Executes the signing command. Pipeline behaviors handle all processing.
+    /// </summary>
+    /// <param name="request"></param>
+    /// <param name="cancellationToken"></param>
+    /// <returns></returns>
+    public Task Handle(SigningCommand request, CancellationToken cancellationToken = default)
+    {
+        return Task.CompletedTask;
+    }
+}