Refactor: Rename OutgoingEmail to SendingEmail

This commit renames and refactors all instances of `OutgoingEmail` to `SendingEmail` across the codebase to improve terminology consistency and align with domain language.

- Renamed classes, interfaces, and records (e.g., `OutgoingEmailPublisher` → `SendingEmailPublisher`, `OutgoingEmailEvent` → `SendingEmailEvent`).
- Updated method signatures, parameters, and return types to use `SendingEmail`.
- Adjusted dependency injection registrations to reflect the new naming.
- Updated mappings in `EmailMappingProfile` to map `SendEmailCommand` to `SendingEmailEvent`.
- Refactored `SendEmailCommand` and its handler to work with `SendingEmailEvent`.
- Updated `EmailsController` to use `SendingEmailEvent` in the `SendEmail` action.
- Refactored integration tests to test `SendingEmailPublisher` and updated test data accordingly.
- Updated log messages, error handling, and comments to reflect the new terminology.
- Revised documentation and utility methods to use `SendingEmailEvent`.

This refactor ensures consistency, improves readability, and reduces ambiguity in the codebase.

src/core/DigitalData.MessagingService.Application/Common/Mappings/EmailMappingProfile.cs

@@ -11,9 +11,9 @@ public class EmailMappingProfile : Profile
 {
     public EmailMappingProfile()
     {
-        // SendEmailCommand -> OutgoingEmailEvent
+        // SendEmailCommand -> SendingEmailEvent
         // Sender is resolved via MediatR in the handler and set separately after mapping.
-        CreateMap<SendEmailCommand, OutgoingEmailEvent>()
+        CreateMap<SendEmailCommand, SendingEmailEvent>()
             .ForMember(dest => dest.Id, opt => opt.MapFrom(_ => Guid.NewGuid()))
             .ForMember(dest => dest.QueuedAt, opt => opt.MapFrom(_ => DateTime.Now))
             .ForMember(dest => dest.Sender, opt => opt.Ignore());