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/infrastructure/DigitalData.MessagingService.Publisher/DependencyInjection.cs

@@ -13,7 +13,7 @@ public static class DependencyInjection
             services.AddRabbitMqConnectionFactory(configure);
 
         // --- Email Queue (RabbitMQ) ---
-        services.AddSingleton<IOutgoingEmailPublisher, OutgoingEmailPublisher>();
+        services.AddSingleton<ISendingEmailPublisher, SendingEmailPublisher>();
 
         return services;
     }