refactor(application): replace GetSenderQuery with GetEmailAccountQuery and add EmailAccountModificationCommand

- Rename GetSenderQuery to GetEmailAccountQuery with updated return type (IEnumerable<EmailAccountDto>)
- Handler now queries IRepository<EmailAccount> directly instead of chaining MediatR dispatches
- Add EmailAccountModificationCommand for create/update/delete operations on email accounts
- Update GetSenderQueryValidator to reference renamed query type
- Remove exclusive Id/Username validation rule (now handled by repo query logic)

src/core/DigitalData.MessagingService.Application/EmailAccounts/Commands/EmailAccountModificationCommand.cs

@@ -0,0 +1,36 @@
+#if NET
+using DigitalData.MessagingService.Application.Common.Dto.EmailAccounts;
+using DigitalData.MessagingService.Application.Common.Interfaces.Repositories;
+using DigitalData.MessagingService.Application.Common.ValueObjects;
+using DigitalData.MessagingService.Domain.Entities;
+using MediatR;
+using System.Security.Principal;
+
+namespace DigitalData.MessagingService.Application.EmailAccounts.Commands;
+
+/// <summary>
+/// DTO for a single email account configuration.
+/// </summary>
+public record EmailAccountModificationCommand : IRequest<(EmailAccount, bool)>
+{
+    public required EmailAccountModificationDto ModifiedAccount { get; init; }
+
+    public required Modification Modification { get; init; }
+}
+
+/// <summary>
+/// 
+/// </summary>
+/// <param name="Repo"></param>
+public class EmailAccountModificationCommandHandler(IRepository<EmailAccount> Repo) : IRequestHandler<EmailAccountModificationCommand, (EmailAccount, bool)>
+{
+    public async Task<(EmailAccount, bool)> Handle(EmailAccountModificationCommand request, CancellationToken cancellationToken)
+    {
+        return request.Modification switch
+        {
+            Modification.Upsert => await Repo.UpsertAsync(a => a.Username == request.ModifiedAccount.Username, request.ModifiedAccount, cancellationToken),
+            _ => throw new NotSupportedException($"Modification type '{request.Modification}' is not supported in email account modification."),
+        };
+    }
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailAccounts/Queries/GetEmailAccountQuery.cs

@@ -0,0 +1,37 @@
+#if NET
+using MediatR;
+using DigitalData.MessagingService.Domain.Entities;
+using DigitalData.MessagingService.Application.Common.Interfaces.Repositories;
+using AutoMapper;
+using DigitalData.MessagingService.Domain.Exceptions;
+using DigitalData.MessagingService.Application.Common.Dto.EmailAccounts;
+
+namespace DigitalData.MessagingService.Application.EmailAccounts.Queries;
+
+public record GetEmailAccountQuery : IRequest<IEnumerable<EmailAccountDto>>
+{
+    public int? Id { get; init; }
+
+    public string? Username { get; init; }
+}
+
+/// <summary>
+/// Handles queries for retrieving email accounts.
+/// </summary>
+/// <param name="Repo"></param>
+/// <param name="Mapper"></param>
+public class GetEmailAccountQueryHandler(IRepository<EmailAccount> Repo, IMapper Mapper) : IRequestHandler<GetEmailAccountQuery, IEnumerable<EmailAccountDto>>
+{
+    public async Task<IEnumerable<EmailAccountDto>> Handle(GetEmailAccountQuery request, CancellationToken cancellationToken)
+    {
+        var accounts = request.Id is null && request.Username is null 
+            ? await Repo.GetAllAsync(cancellationToken) 
+            : await Repo.FindAsync(request.Id is int id ? x => x.Id == id : x => x.Username == request.Username, cancellationToken: cancellationToken);
+
+        if (accounts.Any())
+            return Mapper.Map<IEnumerable<EmailAccountDto>>(accounts);
+        else
+            throw new NotFoundException($"No email account found for the given criteria ({(request.Id is not null ? $"Id: {request.Id}" : $"Username: {request.Username}")}).");
+    }
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailAccounts/Queries/GetSenderQuery.cs

@@ -1,41 +0,0 @@
-#if NET
-using DigitalData.MessagingService.Application.Common.Options;
-using MediatR;
-using Microsoft.Extensions.Logging;
-using Microsoft.Extensions.Options;
-using DigitalData.MessagingService.Domain.Entities;
-
-namespace DigitalData.MessagingService.Application.EmailAccounts.Queries;
-
-public record GetSenderQuery : IRequest<EmailAccount?>
-{
-    public int? Id { get; init; }
-
-    public string? Username { get; init; }
-}
-
-/// <summary>
-/// 
-/// </summary>
-/// <param name="Options"></param>
-/// <param name="Logger"></param>
-public class GetSenderQueryHandler(IOptions<EmailAccountsOptions> Options, ILogger<GetSenderQueryHandler> Logger) : IRequestHandler<GetSenderQuery, EmailAccount?>
-{
-    public Task<EmailAccount?> Handle(GetSenderQuery request, CancellationToken cancellationToken)
-    {
-        var accounts = request.Id is not null
-            ? Options.Value.Accounts.Where(a => a.Id == request.Id)
-            : Options.Value.Accounts.Where(a => a.Username == request.Username);
-
-        if(accounts.Count() > 1)
-        {
-            Logger.LogWarning(
-                "Multiple email accounts found for the given criteria ({Criteria}). Returning the first one.",
-                request.Id is not null ? $"Id: {request.Id}" : $"Username: {request.Username}"
-            );
-        }
-
-        return Task.FromResult(accounts.FirstOrDefault());
-    }
-}
-#endif

src/core/DigitalData.MessagingService.Application/EmailAccounts/Validators/GetSenderQueryValidator.cs

@@ -5,17 +5,13 @@ using FluentValidation;
 namespace DigitalData.MessagingService.Application.EmailAccounts.Validators;
 
 /// <summary>
-/// Validator for <see cref="GetSenderQuery"/>.
+/// Validator for <see cref="GetEmailAccountQuery"/>.
-/// Either <see cref="GetSenderQuery.Id"/> or <see cref="GetSenderQuery.Username"/> must be provided, but not both.
+/// Either <see cref="GetEmailAccountQuery.Id"/> or <see cref="GetEmailAccountQuery.Username"/> must be provided, but not both.
 /// </summary>
-public class GetSenderQueryValidator : AbstractValidator<GetSenderQuery>
+public class GetSenderQueryValidator : AbstractValidator<GetEmailAccountQuery>
 {
     public GetSenderQueryValidator()
     {
-        RuleFor(x => x)
-            .Must(x => (x.Id is not null) ^ (x.Username is not null))
-            .WithMessage("Either Id or Username must be provided, but not both.");
-
         When(x => x.Username is not null, () =>
         {
             RuleFor(x => x.Username)