Update EmailContext mapping to ignore Sender property

The `EmailMappingProfile` class was updated to modify the mapping
configuration between `SendEmailCommand` and `EmailContext`.
A `.ForMember` configuration was added to explicitly ignore the
`Sender` property in the destination (`EmailContext`) during the
mapping process. This ensures that `Sender` is not mapped from
the source object and must be set separately.

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

@@ -13,6 +13,7 @@ public class EmailMappingProfile : Profile
     {
         // SendEmailCommand -> Email
         // Sender is resolved via MediatR in the handler and set separately after mapping.
-        CreateMap<SendEmailCommand, EmailContext>();
+        CreateMap<SendEmailCommand, EmailContext>()
+            .ForMember(dest => dest.Sender, opt => opt.Ignore());
     }
 }