analyzer: strip resolver address from DNS lookup error messages

Wrap user-facing lookup errors through a new formatDNSError helper that
clears net.DNSError.Server so the " on <addr>" suffix no longer leaks the
upstream resolver (e.g. "on 127.0.0.11:53") to end users.

Closes: https://framagit.org/happyDomain/happydeliver/-/work_items/2

pkg/analyzer/dns_resolver.go

@@ -23,9 +23,22 @@ package analyzer
 
 import (
 	"context"
+	"errors"
 	"net"
 )
 
+// formatDNSError renders a resolution error without exposing the upstream
+// resolver address that net.DNSError.Error() normally appends as " on <addr>".
+func formatDNSError(err error) string {
+	var dnsErr *net.DNSError
+	if errors.As(err, &dnsErr) {
+		sanitized := *dnsErr
+		sanitized.Server = ""
+		return sanitized.Error()
+	}
+	return err.Error()
+}
+
 // DNSResolver defines the interface for DNS resolution operations.
 // This interface abstracts DNS lookups to allow for custom implementations,
 // such as mock resolvers for testing or caching resolvers for performance.