Alias DNS record types in SPF checker

Add explicit using aliases for `DnsClient.Protocol.MxRecord` and `DnsClient.Protocol.TxtRecord` in `SpfChecker` to clarify record type references and avoid naming ambiguity.

Author: Taiizor

src/Zetian.AntiSpam/Checkers/SpfChecker.cs

@@ -9,6 +9,8 @@
 using Zetian.AntiSpam.Abstractions;
 using Zetian.AntiSpam.Enums;
 using Zetian.AntiSpam.Models;
+using MxRecord = DnsClient.Protocol.MxRecord;
+using TxtRecord = DnsClient.Protocol.TxtRecord;
 
 namespace Zetian.AntiSpam.Checkers
 {