Support multiple email recipients in email-sending flow

Updated the `IEmailService` interface and related components to
support multiple recipients in the `SendEmailAsync` method.

- Replaced `Recipient` with `Recipients` in `SendEmailCommand`,
  `OutgoingEmailEvent`, and `EmailsController`.
- Updated `SendEmailCommandValidator` to validate a collection
  of recipients, ensuring at least one valid email address.
- Modified `LimilabsEmailService` to handle multiple recipients
  by iterating over the collection and adding each to the email.
- Adjusted `OutgoingEmailConsumer` to process and log multiple
  recipients.
- Updated logging and response structures to reflect the changes.

These changes enable the system to handle emails with multiple
recipients while maintaining proper validation and logging.

src/infrastructure/DigitalData.MessagingService.Infrastructure/Queue/OutgoingEmailConsumer.cs

@@ -49,7 +49,7 @@ public sealed class OutgoingEmailConsumer : IAsyncDisposable
                         // Send email via SMTP (SMTP config is injected in IEmailService via IOptions)
                         await EmailService.SendEmailAsync(
                             oMailEvent.Sender,
-                            oMailEvent.Recipient,
+                            oMailEvent.Recipients,
                             oMailEvent.Subject,
                             oMailEvent.Body,
                             isHtml: oMailEvent.IsHtml);
@@ -65,7 +65,7 @@ public sealed class OutgoingEmailConsumer : IAsyncDisposable
                 }
                 catch (Exception ex)
                 {
-                    logger?.LogError(ex, "Failed to process email [To={To}, Subject={Subject}] message: DeliveryTag={DeliveryTag}. Moving to DLQ (NO retry).", oMailEvent?.Recipient, oMailEvent?.Subject, args.DeliveryTag);
+                    logger?.LogError(ex, "Failed to process email [To={To}, Subject={Subject}] message: DeliveryTag={DeliveryTag}. Moving to DLQ (NO retry).", oMailEvent?.Recipients, oMailEvent?.Subject, args.DeliveryTag);
 
                     // TODO: Error Reporting Strategy
                     // Option 1: Separate RabbitMQ Queue (emailprofiler.errors)

src/infrastructure/DigitalData.MessagingService.Infrastructure/Services/LimilabsEmailService.cs

@@ -27,7 +27,7 @@ public class LimilabsEmailService(
         Encoding.RegisterProvider(CodePagesEncodingProvider.Instance);
     }
 
-    public async Task SendEmailAsync(EmailAccountDto from, string to, string subject, string body, bool isHtml = true, CancellationToken cancellationToken = default)
+    public async Task SendEmailAsync(EmailAccountDto from, IEnumerable<string> recipients, string subject, string body, bool isHtml = true, CancellationToken cancellationToken = default)
     {
         using var smtp = new Smtp();
 
@@ -37,7 +37,10 @@ public class LimilabsEmailService(
 
             var builder = new MailBuilder();
             builder.From.Add(new MailBox(from.Username));
-            builder.To.Add(new MailBox(to));
+
+            foreach (var recipient in recipients)
+                builder.To.Add(new MailBox(recipient));
+
             builder.Subject = subject;
 
             if (isHtml)