feat(application): add ToSender helper for email accounts

Adds GetEmailAccountQueryExtensions.ToSender to build a sender query from an email address, and documents its use on PublishEmailViaSmtpCommand.Sender.

src/core/DigitalData.MessagingService.Application/EmailSending/Commands/GetEmailAccountQueryExtensions.cs

@@ -0,0 +1,16 @@
+#if NET
+using DigitalData.MessagingService.Application.EmailAccounts.Queries;
+
+namespace DigitalData.MessagingService.Application.EmailSending.Commands;
+
+public static class GetEmailAccountQueryExtensions
+{
+    public static GetEmailAccountQuery ToSender(string emailAddress)
+    {
+        return new GetEmailAccountQuery
+        {
+            Username = emailAddress
+        };
+    }
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailSending/Commands/PublishEmailViaSmtpCommand.cs

@@ -17,6 +17,10 @@ namespace DigitalData.MessagingService.Application.EmailSending.Commands;
 /// </summary>
 public record PublishEmailViaSmtpCommand : IRequest<Guid>
 {
+    /// <summary>
+    /// Specifies the sender's email account to resolve, identified by either its Id or Username.
+    /// Use <see cref="GetEmailAccountQueryExtensions.ToSender(string)"/> to create a query from an email address.
+    /// </summary>
     public required GetEmailAccountQuery Sender { get; init; }
 
     /// <summary>