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/core/DigitalData.MessagingService.Application/Common/Models/MailSearch/MailSearchFilter.cs

@@ -19,12 +19,6 @@ public record MailSearchFilter
     /// </summary>
     public bool UnseenOnly { get; init; } = false;
 
-    /// <summary>
-    /// When <see langword="true"/>, only messages that carry at least one attachment are returned.
-    /// Filtered client-side after fetching the message envelope; does not affect the IMAP SEARCH query.
-    /// </summary>
-    public bool HasAttachments { get; init; } = false;
-
     /// <summary>
     /// When <see langword="true"/>, attachment data is included in the results; otherwise attachments are omitted.
     /// Defaults to <see langword="false"/>.