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/SendSignedMailBehavior.cs

@@ -17,7 +17,7 @@ namespace EnvelopeGenerator.Application.Signatures.Behaviors;
 /// Pipeline behavior that sends signed mail notification.
 /// Executes LAST in the signing process - only if all previous behaviors succeed.
 /// </summary>
-public class SendSignedMailBehavior : IPipelineBehavior<SignCommand, Unit>
+public class SendSignedMailBehavior : IPipelineBehavior<SigningCommand, Unit>
 {
     private readonly IRepository<EmailTemplate> _tempRepo;
     private readonly IRepository<EmailOut> _emailOutRepo;
@@ -50,7 +50,7 @@ public class SendSignedMailBehavior : 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)
     {
         var placeHolders = CreatePlaceHolders(request);
 
@@ -82,7 +82,7 @@ public class SendSignedMailBehavior : IPipelineBehavior<SignCommand, Unit>
         return await next(cancellationToken);
     }
 
-    private Dictionary<string, string> CreatePlaceHolders(SignCommand request)
+    private Dictionary<string, string> CreatePlaceHolders(SigningCommand request)
     {
         var placeHolders = new Dictionary<string, string>()
         {