Refactor namespaces and remove Abstraction project

Replaced `DigitalData.MessagingService.Abstraction` with
`DigitalData.MessagingService.Application.Common.Dto` and
`DigitalData.MessagingService.Application.Common.Dto.MailSearch`
to improve modularity and organization.

Removed the `Abstraction` project and updated all references
to use the `Application` project. Updated namespaces, `using`
directives, and dependencies across the codebase.

Refactored interfaces, commands, queries, validators, and
services to use the new DTOs. Updated RabbitMQ integration,
AutoMapper profiles, background services, and tests to align
with the new structure.

Performed general cleanup by removing redundant `using`
directives and obsolete references.

src/core/DigitalData.MessagingService.Application/Common/Dto/EmailContext.cs

@@ -0,0 +1,55 @@
+namespace DigitalData.MessagingService.Application.Common.Dto;
+
+public record EmailContext
+{
+#if NETFRAMEWORK
+    public EmailAccountDto Sender { get; set; } = null!;
+#else
+    public required EmailAccountDto Sender { get; init; }
+#endif
+
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+
+#if NETFRAMEWORK
+    public IEnumerable<string> Recipients { get; set; } = null!;
+#else
+    public required IEnumerable<string> Recipients { get; init; }
+#endif
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+
+#if NETFRAMEWORK
+    public string Subject { get; set; } = null!;
+#else
+    public required string Subject { get; init; }
+#endif
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+#if NETFRAMEWORK
+    public string Body { get; set; } = null!;
+#else
+    public required string Body { get; init; }
+#endif
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+#if NETFRAMEWORK
+    public bool IsHtml { get; set; } = true;
+#else
+    public bool IsHtml { get; init; } = true;
+#endif
+
+    /// <summary>
+    /// Optional list of attachments to include with the email.
+    /// Each entry may carry its content as a byte array (<see cref="EmailAttachmentDto.Content"/>)
+    /// or reference a file on disk via <see cref="EmailAttachmentDto.FilePath"/>.
+    /// </summary>
+    public IEnumerable<EmailAttachmentDto> Attachments { get; set; } = [];
+}