Refactor email handling for improved structure

Refactored `Email` and `SendingEmailEvent` to use `record` types, consolidating email-related data into the `Email` class. Updated `SendEmailCommand` to return a `Guid` and simplified mapping logic in `EmailMappingProfile`. Adjusted `SendEmailCommandHandler` to construct `SendingEmailEvent` manually.

Updated `SendingEmailConsumer`, `EmailsController`, and `EmailSender` to reflect the new structure. Removed the old `Email` implementation. Improved logging to reference the `Mail` property.

Revised tests to align with the new structure, ensuring immutability and better separation of concerns.

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

@@ -11,11 +11,8 @@ public class EmailMappingProfile : Profile
 {
     public EmailMappingProfile()
     {
-        // SendEmailCommand -> SendingEmailEvent
+        // SendEmailCommand -> Email
         // Sender is resolved via MediatR in the handler and set separately after mapping.
-        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());
+        CreateMap<SendEmailCommand, Email>();
     }
 }