Added XML documentation comments to the `NotFoundException` class and its constructors. This improves code readability and provides clear descriptions for developers using this exception.
Removed unnecessary using directives in `UpdateEmailTemplateCommandHandler.cs`. Added a new `BadRequestException` class in `BadRequestException.cs` to handle bad request scenarios with customizable error messages.
- Updated EmailTemplateDto to allow mutable Body and Subject properties.
- Implemented IRequest interface in UpdateEmailTemplateCommand for MediatR.
- Enhanced error handling in EmailTemplateController with NotFoundException.
- Introduced UpdateEmailTemplateCommandHandler for processing update commands.
- Added NotFoundException class for improved error handling.
