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/Common/Interfaces/IImapEmailService.cs

@@ -1,3 +1,4 @@
+#if NET
 using DigitalData.MessagingService.Abstraction;
 using DigitalData.MessagingService.Application.Common.Models.MailSearch;
 
@@ -31,3 +32,4 @@ public interface IImapEmailService
         string folder = "INBOX",
         CancellationToken cancellationToken = default);
 }
+#endif

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

@@ -1,3 +1,4 @@
+#if NET
 using AutoMapper;
 using DigitalData.MessagingService.Application.EmailSending.Commands;
 using DigitalData.MessagingService.Abstraction;
@@ -18,3 +19,4 @@ public class EmailMappingProfile : Profile
             .ForMember(dest => dest.Attachments, opt => opt.MapFrom(src => src.Attachments));
     }
 }
+#endif

src/core/DigitalData.MessagingService.Application/Common/Models/MailSearch/DateFilter.cs

@@ -1,3 +1,4 @@
+#if NET
 namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
 
 /// <summary>
@@ -11,3 +12,4 @@ namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
 /// <param name="After">Earliest date to include (inclusive). Maps to IMAP <c>SINCE</c>.</param>
 /// <param name="Before">Latest date to include (inclusive). Maps to IMAP <c>BEFORE</c> (next day is used internally).</param>
 public record DateFilter(DateTime? After = null, DateTime? Before = null);
+#endif

src/core/DigitalData.MessagingService.Application/Common/Models/MailSearch/MailSearchFilter.cs

@@ -1,4 +1,5 @@
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+#if NET
+namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
 
 /// <summary>
 /// Describes all criteria and options used when searching an IMAP mailbox.
@@ -75,4 +76,5 @@ public record MailSearchFilter
     /// See <see cref="DateFilter"/> for rules between its fields.
     /// </summary>
     public DateFilter? Date { get; init; } = null;
-}
+}
+#endif

src/core/DigitalData.MessagingService.Application/Common/Models/MailSearch/UidFilter.cs

@@ -1,4 +1,5 @@
-namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
+#if NET
+namespace DigitalData.MessagingService.Application.Common.Models.MailSearch;
 
 /// <summary>
 /// Constrains the search to a specific UID or a UID range.
@@ -11,4 +12,5 @@
 /// <param name="Min">Lower bound of the UID range (inclusive). Ignored when <see cref="Absolute"/> is set.</param>
 /// <param name="Max">Upper bound of the UID range (inclusive). Ignored when <see cref="Absolute"/> is set.</param>
 /// <param name="Absolute">Exact UID to match. When set, <see cref="Min"/> and <see cref="Max"/> must be <see langword="null"/>.</param>
-public record UidFilter(long? Min = null, long? Max = null, long? Absolute = null);
+public record UidFilter(long? Min = null, long? Max = null, long? Absolute = null);
+#endif

src/core/DigitalData.MessagingService.Application/Common/Options/EmailAccountsOptions.cs

@@ -1,4 +1,5 @@
-using DigitalData.MessagingService.Abstraction;
+#if NET
+using DigitalData.MessagingService.Abstraction;
 
 namespace DigitalData.MessagingService.Application.Common.Options;
 
@@ -21,3 +22,4 @@ public class EmailAccountsOptions
     /// </summary>
     public int SyncIntervalSeconds { get; init; } = 300;
 }
+#endif