fix: hold/resume invite SDP direction negotiation and IP handling

[vyntro]

Fix for the issue #125

I have observed the below three issue when i try to use the call hold on functionality.

1. SDP Direction Handling Issue
When a hold re-INVITE is received with a=sendonly in the SDP, the answer is incorrectly generated with a=sendrecv instead of a=recvonly. This causes the remote side to stop sending RTP/audio after resume, even though the call remains up. Correct SDP direction negotiation is critical: the answer must reflect the proper direction (sendonly → recvonly, recvonly → sendonly, etc.).

2. SDP Parser Bug (No Trailing Newline)
If the direction attribute (a=sendonly, a=recvonly, etc.) is the last line in the SDP and there is no trailing newline, the parser skips this line. As a result, the direction is not processed, and the answer defaults to a=sendrecv This causes incorrect media negotiation and RTP/audio failures after hold/resume.

3. IP Handling Issue
When answering a hold/resume re-INVITE, the 200 OK SDP sometimes uses c=IN IP6 :: or c=IN IP4 0.0.0.0 instead of the configured IPv4 address or ExternalIP. This can cause the remote side to reject the media or stop sending RTP.

Impact

• RTP/audio is not received after hold/resume due to incorrect SDP direction and IP handling.
• Calls remain up, but media is broken.
• SIP traces show 200 OK answers with wrong direction and/or IP.

Expected

• SDP direction attributes are always honored, even if they are the last line without a newline.
• Answers to hold re-INVITE use a=recvonly when offer is a=sendonly.
• 200 OK SDP uses the correct IPv4 address (ExternalIP) in the c= line.

Actual

• SDP direction is skipped or defaults to a=sendrecv.
• 200 OK SDP uses c=IN IP6 :: or c=IN IP4 0.0.0.0.
• RTP/audio not received after resume

SIP trace (abridged)

INVITE (hold): SDP last line `a=sendonly` (no newline)
200 OK: SDP contains `a=sendrecv` (should be `recvonly`), c=IN IP6 :: or c=IN IP4 0.0.0.0
ACK
INVITE (resume): SDP `a=sendrecv`
200 OK: SDP `a=sendrecv`, c=IN IP6 :: or c=IN IP4 0.0.0.0
ACK
RTP/audio not received from remote