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/HistoryBehavior.cs

@@ -10,7 +10,7 @@ namespace EnvelopeGenerator.Application.Signatures.Behaviors;
 /// Pipeline behavior that records history.
 /// Executes third in the signing process.
 /// </summary>
-public class HistoryBehavior : IPipelineBehavior<SignCommand, Unit>
+public class HistoryBehavior : IPipelineBehavior<SigningCommand, Unit>
 {
     private readonly ISender _sender;
 
@@ -30,7 +30,7 @@ public class HistoryBehavior : 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.EnvelopeReceiver.Receiver is null)
             throw new InvalidOperationException($"Receiver information is missing in the notification. SignCommand:\n {request.ToJson(Format.Json.ForDiagnostics)}");