perf(dot): implement connection pooling for improved performance

Implement TCP/TLS connection pooling for DoT resolver to match DoQ
performance. Previously, DoT created a new TCP/TLS connection for every
DNS query, incurring significant TLS handshake overhead. Now connections are
reused across queries, eliminating this overhead for subsequent requests.

The implementation follows the same pattern as DoQ, using parallel dialing
and connection pooling to achieve comparable performance characteristics.

doq.go

@@ -21,6 +21,10 @@ type doqResolver struct {
 }
 
 func (r *doqResolver) Resolve(ctx context.Context, msg *dns.Msg) (*dns.Msg, error) {
+	if err := validateMsg(msg); err != nil {
+		return nil, err
+	}
+
 	// Get the appropriate connection pool based on DNS type and IP stack
 	dnsTyp := uint16(0)
 	if msg != nil && len(msg.Question) > 0 {