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/EmailSenderUrlOverloadTests.cs

@@ -55,14 +55,12 @@ public sealed class EmailSenderUrlOverloadTests
     [Fact]
     public void Send_AfterUrlOverloadConnection_DoesNotThrow()
     {
-        var email = new OutgoingEmailEvent
+        var email = new Email
         {
-            Id = Guid.NewGuid(),
             Recipient = "url-overload-test@example.com",
             Subject = "URL Overload Integration Test",
             Body = "<p>Sent after URL-based connection.</p>",
-            IsHtml = true,
-            QueuedAt = DateTime.Now
+            IsHtml = true
         };
 
         // Connection was established via Action<> overload in the fixture;