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

@@ -8,7 +8,7 @@ namespace EnvelopeGenerator.Application.Signatures.Commands;
 /// <summary>
 /// Command to sign a document by a receiver.
 /// </summary>
-public record SignCommand : EnvelopeReceiverQueryBase, IRequest
+public record SigningCommand : EnvelopeReceiverQueryBase, IRequest
 {
     private EnvelopeReceiverDto? _envelopeReceiver;
 
@@ -37,7 +37,7 @@ public record SignCommand : EnvelopeReceiverQueryBase, IRequest
 /// 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<SignCommand>
+public class SignCommandHandler : IRequestHandler<SigningCommand>
 {
     /// <summary>
     /// Executes the signing command. Pipeline behaviors handle all processing.
@@ -45,7 +45,7 @@ public class SignCommandHandler : IRequestHandler<SignCommand>
     /// <param name="request"></param>
     /// <param name="cancellationToken"></param>
     /// <returns></returns>
-    public Task Handle(SignCommand request, CancellationToken cancellationToken = default)
+    public Task Handle(SigningCommand request, CancellationToken cancellationToken = default)
     {
         return Task.CompletedTask;
     }