fix: bind SIP media sockets to signaling interface

[AlexanderMatveev]

Summary

Adds an opt-in way to bind SIP media (RTP) sockets to a specific local interface, instead of always listening on 0.0.0.0.

This is a rebased revival of #659 (closed without merge), updated for review feedback: do not change the default bind address.

Problem

On multi-homed hosts, SIP signaling may use one interface/IP while RTP sockets bound to 0.0.0.0 egress via another. Some SIP providers require signaling and media to originate from the same IP.

nat_1_to_1_ip / media_nat_1_to_1_ip only affect announce; they do not bind the RTP socket. listen_ip only covers SIP signaling.

Change

• Add BindIP to MediaOptions
• Add optional config media_listen_ip
• Resolve RTP bind IP only from explicit config: media_listen_ip → specific listen_ip → else 0.0.0.0
• No inference from SignalingIPLocal (announce/discovery value; unsafe as a default bind)
• Unit tests for resolver + explicit BindIP socket bind

Operators who need same-IP media/signaling on multi-homed hosts set media_listen_ip (or a specific listen_ip). Everyone else keeps today’s 0.0.0.0 behavior.

Test plan

• go test ./pkg/sip/ -run 'TestParseSpecificListenIP|TestResolveMediaBindIP|TestMediaPortBindIP'
• Optional: multi-homed smoke with media_listen_ip set to the signaling NIC

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.04%. Comparing base (0460b40) to head (a96065e).
⚠️ Report is 346 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #784      +/-   ##
==========================================
+ Coverage   65.25%   67.04%   +1.78%     
==========================================
  Files          51       41      -10     
  Lines        6588     8122    +1534     
==========================================
+ Hits         4299     5445    +1146     
- Misses       1915     2183     +268     
- Partials      374      494     +120     

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