[Fix] Fix TCP DNS uninitialized memory leak

[Fix] Fix TCP DNS uninitialized memory leak

When rescheduling a DNS request from UDP to TCP, the code was using
req->packet_len (allocated buffer size) instead of req->pos (actual
packet size) to copy and send the DNS packet. This caused random
garbage from uninitialized memory to be appended to TCP DNS queries.

The bug was particularly noticeable with short queries like TXT records,
where the allocated buffer could be 2-3x larger than the actual packet.