SPNEGO/Kerberos: fails on accept-completed without AP-REP (no mutual auth)

When using SPNEGO/Kerberos (Negotiate) without requesting mutual authentication, the client receives negTokenTarg with negResult=accept-completed and no AP-REP, which is correct per server behavior. However, sspi-rs still tries to extract an AP-REP and fails in extract_ap_rep_from_neg_token_targ, aborting the handshake.

Start at extract_ap_rep_from_neg_token_targ and trace the SPNEGO/Kerberos handshake path for negResult=accept-completed. Verify the no-mutual-authentication case where the response has no AP-REP, and consider the issue complete when that handshake succeeds without aborting while AP-REP handling still works when present.

Written by the indexing model from the issue text.