Add support for email attachments in SendEmailCommand

Enhanced the email-sending workflow to support attachments:
- Updated `SendEmailCommand` with an `Attachments` property.
- Added `WithAttachments` method to handle attachment initialization.
- Modified `EmailsController` to accept file uploads via `IFormFileCollection`.
- Implemented `BuildAttachmentsAsync` to process uploaded files.
- Updated `EmailMappingProfile` to map `Attachments` to `EmailContext`.
- Adjusted `SendEmail` endpoint to consume `multipart/form-data`.
- Enhanced `SendEmail` response to include the queued event ID.
- Updated project file to include Swagger infrastructure folder.

These changes enable handling of email attachments and improve API functionality.

src/core/DigitalData.MessagingService.Application/Common/Mappings/EmailMappingProfile.cs

@@ -14,6 +14,7 @@ public class EmailMappingProfile : Profile
         // SendEmailCommand -> Email
         // Sender is resolved via MediatR in the handler and set separately after mapping.
         CreateMap<SendEmailCommand, EmailContext>()
-            .ForMember(dest => dest.Sender, opt => opt.Ignore());
+            .ForMember(dest => dest.Sender, opt => opt.Ignore())
+            .ForMember(dest => dest.Attachments, opt => opt.MapFrom(src => src.Attachments));
     }
 }

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

@@ -3,6 +3,7 @@ using DigitalData.MessagingService.Application.EmailAccount.Queries;
 using DigitalData.MessagingService.Domain.Exceptions;
 using DigitalData.MessagingService.Abstraction;
 using MediatR;
+using System.Text.Json.Serialization;
 
 namespace DigitalData.MessagingService.Application.EmailSending.Commands;
 
@@ -32,6 +33,16 @@ public record SendEmailCommand : IRequest<Guid>
     /// Is HTML email (default: true)
     /// </summary>
     public bool IsHtml { get; init; } = true;
+
+    [JsonIgnore]
+    internal IEnumerable<EmailAttachmentContext> Attachments { get; private init; } = [];
+
+    /// <summary>
+    /// Returns a new command instance with the supplied attachments.
+    /// Called by the controller after resolving uploaded files.
+    /// </summary>
+    public SendEmailCommand WithAttachments(IEnumerable<EmailAttachmentContext> attachments)
+        => this with { Attachments = attachments };
 }
 
 /// <summary>