Remove HasAttachments filter; enhance FetchEmails response

The `MailSearchFilter` class was updated to remove the `HasAttachments` property, simplifying the filtering logic. Corresponding client-side filtering logic in `LimilabsImapEmailService` was also removed. The IMAP search query now defaults to `Expression.All` when no criteria are provided.

The `FetchEmails` method in `EmailController` was enhanced with a new optional `firstHtmlBodyOnly` parameter. This allows returning only the HTML body of the first email or a `404 Not Found` response if no emails match the criteria. These changes improve flexibility and simplify the codebase.

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

@@ -78,9 +78,16 @@ public class EmailController(IMediator mediator) : ControllerBase
     [ProducesResponseType(StatusCodes.Status200OK)]
     [ProducesResponseType(StatusCodes.Status400BadRequest)]
     [ProducesResponseType(StatusCodes.Status404NotFound)]
-    public async Task<IActionResult> FetchEmails([FromQuery] FetchEmailsQuery query, CancellationToken cancellationToken = default)
+    public async Task<IActionResult> FetchEmails([FromQuery] FetchEmailsQuery query, [FromQuery] bool firstHtmlBodyOnly = false, CancellationToken cancellationToken = default)
     {
         var emails = await mediator.Send(query, cancellationToken);
+
+        if(!emails.Any())
+            return NotFound("No emails found matching the specified criteria.");
+
+        if (firstHtmlBodyOnly && emails.FirstOrDefault()?.HtmlBody is string htmlBody)
+            return Ok(htmlBody);
+
         return Ok(emails);
     }