Refactor EmailSender to use new Email abstraction

Introduced a new `Email` record to simplify the representation of outgoing email messages. Updated the `EmailSender.Send` method to accept `Email` instead of `OutgoingEmailEvent`, with a `ToEvent()` method handling the conversion internally.

Refactored test cases to use the `Email` record, removing redundant properties (`Id` and `QueuedAt`) that are now auto-generated. Updated XML documentation to reflect these changes. Adjusted namespaces and added necessary `using` directives for proper referencing.

These changes improve code readability, maintainability, and centralize the mapping logic for outgoing email events.

tests/DigitalData.MessagingService.Tests/Integration/EmailSenderTests.cs

@@ -75,14 +75,12 @@ public sealed class EmailSenderTests
     [Fact]
     public void Send_WithValidEmail_DoesNotThrow()
     {
-        var email = new OutgoingEmailEvent
+        var email = new Email
         {
-            Id = Guid.NewGuid(),
             Recipient = "hakanttek@gmail.com",
             Subject = "EmailSender.Send Integration Test",
             Body = "<p>Sent via EmailSender static client.</p>",
             IsHtml = true,
-            QueuedAt = DateTime.Now
         };
 
         var exception = Record.Exception(() => EmailSender.Send(email));
@@ -93,14 +91,12 @@ public sealed class EmailSenderTests
     [Fact]
     public void Send_WithPlainTextBody_DoesNotThrow()
     {
-        var email = new OutgoingEmailEvent
+        var email = new Email
         {
-            Id = Guid.NewGuid(),
             Recipient = "hakanttek@gmail.com",
             Subject = "Plain Text Test",
             Body = "This is a plain text email.",
-            IsHtml = false,
-            QueuedAt = DateTime.Now
+            IsHtml = false
         };
 
         var exception = Record.Exception(() => EmailSender.Send(email));