Replaced `DigitalData.MessagingService.Abstraction` with
`DigitalData.MessagingService.Application.Common.Dto` and
`DigitalData.MessagingService.Application.Common.Dto.MailSearch`
to improve modularity and organization.
Removed the `Abstraction` project and updated all references
to use the `Application` project. Updated namespaces, `using`
directives, and dependencies across the codebase.
Refactored interfaces, commands, queries, validators, and
services to use the new DTOs. Updated RabbitMQ integration,
AutoMapper profiles, background services, and tests to align
with the new structure.
Performed general cleanup by removing redundant `using`
directives and obsolete references.
