Refactor Email to EmailContext across codebase

Replaced the `Email` record with the new `EmailContext` record to introduce additional context and functionality in email handling. Updated property definitions to distinguish between .NET Framework (`set`) and other frameworks (`init`).

Modified `SendingEmailEvent` to use `EmailContext` for the `Mail` property. Updated mappings in `EmailMappingProfile` to map `SendEmailCommand` to `EmailContext`. Adjusted `SendEmailCommandHandler` to use `EmailContext` when mapping requests.

Refactored `EmailSender` to use `EmailContext` in its `Send` method, including updates to method signatures and documentation. Updated all related test classes (`EmailSenderTests`, `EmailSenderUrlOverloadTests`, `SendingEmailPublisherTests`) to validate the behavior of `EmailContext`, ensuring consistency and thorough testing of the transition.

These changes ensure compatibility across frameworks and improve the maintainability of the email handling process.

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

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