Add multi-targeting support for .NET frameworks

Introduced conditional compilation using `#if NET` directives to
enable support for multiple frameworks (`net462`, `net480`, and
`net8.0`). Updated the project file to support multi-targeting
and added framework-specific dependencies conditionally.

Refactored namespaces, interfaces, classes, validators, and
handlers to ensure compatibility with targeted frameworks.
Enhanced dependency injection setup and adjusted logic in
`GetSenderQueryHandler` for framework-specific behavior.

Ensured consistency and maintainability by wrapping framework-
specific code blocks with `#if NET` directives.

src/core/DigitalData.MessagingService.Application/EmailReceiving/Commands/MarkEmailAsSeenCommand.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Application.Common.Interfaces;
 using DigitalData.MessagingService.Application.EmailAccount.Queries;
 using DigitalData.MessagingService.Domain.Exceptions;
@@ -40,3 +41,4 @@ public class MarkEmailAsSeenCommandHandler(
         await ImapService.MarkAsSeenAsync(account, request.Uid, request.Folder, cancellationToken);
     }
 }
+#endif

src/core/DigitalData.MessagingService.Application/EmailReceiving/Queries/FetchEmailsQuery.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Abstraction;
 using DigitalData.MessagingService.Application.Common.Interfaces;
 using DigitalData.MessagingService.Application.Common.Models.MailSearch;
@@ -43,3 +44,4 @@ public class FetchEmailsQueryHandler(
             cancellationToken);
     }
 }
+#endif

src/core/DigitalData.MessagingService.Application/EmailReceiving/Validators/DateFilterValidator.cs

@@ -1,4 +1,5 @@
-using DigitalData.MessagingService.Application.Common.Models.MailSearch;
+#if NET
+using DigitalData.MessagingService.Application.Common.Models.MailSearch;
 using FluentValidation;
 
 namespace DigitalData.MessagingService.Application.EmailReceiving.Validators;
@@ -34,4 +35,5 @@ public class DateFilterValidator : AbstractValidator<DateFilter>
             .WithMessage("Date.After must be earlier than Date.Before.")
             .When(x => x.After is not null && x.Before is not null);
     }
-}
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailReceiving/Validators/FetchEmailsQueryValidator.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Application.EmailReceiving.Queries;
 using FluentValidation;
 
@@ -19,4 +20,5 @@ public class FetchEmailsQueryValidator : AbstractValidator<FetchEmailsQuery>
             .WithMessage("Mail search filter must not be null.")
             .SetValidator(new MailSearchFilterValidator());
     }
-}
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailReceiving/Validators/MailSearchFilterValidator.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Application.Common.Models.MailSearch;
 using FluentValidation;
 
@@ -48,4 +49,5 @@ public class MailSearchFilterValidator : AbstractValidator<MailSearchFilter>
             .SetValidator(new DateFilterValidator()!)
             .When(x => x.Date is not null);
     }
-}
+}
+#endif

src/core/DigitalData.MessagingService.Application/EmailReceiving/Validators/UidFilterValidator.cs

@@ -1,4 +1,5 @@
-using DigitalData.MessagingService.Application.Common.Models.MailSearch;
+#if NET
+using DigitalData.MessagingService.Application.Common.Models.MailSearch;
 using FluentValidation;
 
 namespace DigitalData.MessagingService.Application.EmailReceiving.Validators;
@@ -45,4 +46,5 @@ public class UidFilterValidator : AbstractValidator<UidFilter>
             .WithMessage("Uid.Min must be less than or equal to Uid.Max.")
             .When(x => x.Min is not null && x.Max is not null);
     }
-}
+}
+#endif