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.

AGENTS.md

@@ -330,7 +330,7 @@ return Accepted(); // HTTP 202 - command queued for processing
                arguments: null);
        }
        
-       public async Task EnqueueAsync(OutgoingEmail email, CancellationToken cancellationToken)
+       public async Task EnqueueAsync(SendingEmail email, CancellationToken cancellationToken)
        {
            var json = JsonSerializer.Serialize(email);
            var body = Encoding.UTF8.GetBytes(json);
@@ -347,7 +347,7 @@ return Accepted(); // HTTP 202 - command queued for processing
            await Task.CompletedTask;
        }
        
-       public async Task<OutgoingEmail?> DequeueAsync(CancellationToken cancellationToken)
+       public async Task<SendingEmail?> DequeueAsync(CancellationToken cancellationToken)
        {
            var result = _channel.BasicGet(QueueName, autoAck: false);
            
@@ -355,7 +355,7 @@ return Accepted(); // HTTP 202 - command queued for processing
                return null;
            
            var json = Encoding.UTF8.GetString(result.Body.ToArray());
-           var email = JsonSerializer.Deserialize<OutgoingEmail>(json);
+           var email = JsonSerializer.Deserialize<SendingEmail>(json);
            
            _channel.BasicAck(result.DeliveryTag, false);