Refactor: Consolidate Publisher.Abstraction into Abstraction

The `DigitalData.MessagingService.Publisher.Abstraction` project has been removed, and its functionality has been merged into a new project named `DigitalData.MessagingService.Abstraction`.

- Updated namespaces from `Publisher.Abstraction` to `Abstraction` across all relevant files, including DTOs, interfaces, and classes.
- Modified the solution file to remove `Publisher.Abstraction` and add `Abstraction`, updating solution configurations and nested project mappings.
- Replaced project references to `Publisher.Abstraction` with `Abstraction` in all affected project files.
- Updated tests and integration tests to reflect the namespace and project changes.
- Refactored application-level files such as `EmailMappingProfile` and `DependencyInjection.cs` to use the new namespace.

This refactor simplifies the project structure and ensures consistency across the solution.

src/core/DigitalData.MessagingService.Abstraction/OutgoingEmailCreateDto.cs

@@ -0,0 +1,26 @@
+namespace DigitalData.MessagingService.Abstraction;
+
+public record OutgoingEmailCreateDto
+{
+    /// <summary>
+    /// Recipient email address
+    /// </summary>
+    public string Recipient { get; set; } = null!;
+
+    /// <summary>
+    /// Email subject
+    /// </summary>
+    public string Subject { get; set; } = null!;
+
+    /// <summary>
+    /// Email body (HTML or plain text)
+    /// </summary>
+    public string Body { get; set; } = null!;
+
+    /// <summary>
+    /// Is HTML email (default: true)
+    /// </summary>
+    public bool IsHtml { get; set; } = true;
+
+    public DateTime QueuedAt { get; set; }
+}