After a blind transfer, the PBX's REFER progress NOTIFYs go unanswered #24

Summary

This is a follow-up to #14 / #18, found while live-testing the merged master against a VitalPBX (Asterisk 20.14) lab PBX.

With #18, BlindTransferAsync hangs up the original leg as soon as the REFER is accepted (202). But Asterisk reports the transfer's progress with in-dialog NOTIFYs (Event: refer, Content-Type: message/sipfrag), sent about 10 ms after the 202. The last one carries SIP/2.0 200 OK and Subscription-State: terminated. By the time they arrive the dialog is gone, so nothing answers them, and the PBX retransmitted each one for ~32 s. The live trace had 3 NOTIFYs and 6 retransmissions. Before #18 the leg stayed up and SIPUserAgent answered them in the dialog.

Expected

After the REFER is accepted, BlindTransferAsync waits briefly for the final progress NOTIFY (sipfrag status 200 or higher), which SIPUserAgent answers in the dialog, and then hangs up. If none arrives within a few seconds (not every PBX sends them), it hangs up anyway.