Refactor: Rename OutgoingEmail to SendingEmail

This commit renames and refactors all instances of `OutgoingEmail` to `SendingEmail` across the codebase to improve terminology consistency and align with domain language.

- Renamed classes, interfaces, and records (e.g., `OutgoingEmailPublisher` → `SendingEmailPublisher`, `OutgoingEmailEvent` → `SendingEmailEvent`).
- Updated method signatures, parameters, and return types to use `SendingEmail`.
- Adjusted dependency injection registrations to reflect the new naming.
- Updated mappings in `EmailMappingProfile` to map `SendEmailCommand` to `SendingEmailEvent`.
- Refactored `SendEmailCommand` and its handler to work with `SendingEmailEvent`.
- Updated `EmailsController` to use `SendingEmailEvent` in the `SendEmail` action.
- Refactored integration tests to test `SendingEmailPublisher` and updated test data accordingly.
- Updated log messages, error handling, and comments to reflect the new terminology.
- Revised documentation and utility methods to use `SendingEmailEvent`.

This refactor ensures consistency, improves readability, and reduces ambiguity in the codebase.

src/presentation/DigitalData.MessagingService.API/Controllers/EmailsController.cs

@@ -23,11 +23,11 @@ public class EmailsController(IMediator mediator) : ControllerBase
     [ProducesResponseType(StatusCodes.Status400BadRequest)]
     public async Task<IActionResult> SendEmail([FromBody] SendEmailCommand command, CancellationToken cancellationToken)
     {
-        var outgoingEmailEvent = await mediator.Send(command, cancellationToken);
+        var sendingEmailEvent = await mediator.Send(command, cancellationToken);
 
         return Accepted(new
         {
-            outgoingEmailEvent.Id,
+            sendingEmailEvent.Id,
         });
     }
 }