fix(sip): send 487 Request Terminated after OK (CANCEL) for pending INVITE

[mubashiroliyantakath]

When a CANCEL is received for an INVITE that hasn't been answered, the SIP gateway now correctly sends a 487 response to the original INVITE. This ensures proper SIP transaction termination and avoids issues where upstream proxies (e.g., Brekeke) keep the call in an INVITE state due to the missing final response.

closes #371

[CLAassistant]

All committers have signed the CLA.

[dennwc]

Thank you for a PR! It looks good, except that inviteTx must be accessed under a mutex only.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 62.56%. Comparing base (0460b40) to head (6463a66).
Report is 100 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/sip/inbound.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #372      +/-   ##
==========================================
- Coverage   65.25%   62.56%   -2.69%     
==========================================
  Files          51       32      -19     
  Lines        6588     6321     -267     
==========================================
- Hits         4299     3955     -344     
- Misses       1915     1992      +77     
  Partials      374      374              

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[mubashiroliyantakath]

@dennwc thanks for the suggestion. Fixed it.

[zusrut]

@mubashiroliyantakath you also need to check if the INVITE already got final response and in this case send BYE to terminate the session instead of 487.

@dennwc livekit/sip does not support sip signaling and requires deep changes. also i believe sipgo lib already cover some cases by its own - no need to implement it on such low level

[dennwc]

@zusrut although you are right about sipgo supporting dialogs, we do use an older version of it. We need to migrate to a newer version before we can rely on it.

[alastor09]

@dennwc any other change you expect to be made before this can be merged? or any concerns?