fix(sip): route in-dialog requests through the proxy's Record-Route path, not back at LiveKit-SIP

[rkfshakti]

Summary

TransferSipParticipant fails against a multi-hop proxy (Avaya Session Manager): the out-of-dialog REFER gets a Route header pointing at LiveKit-SIP's own address instead of the proxy's Record-Route path. The proxy dutifully forwards to LiveKit-SIP, which doesn't recognize it as a valid inbound request and rejects with 405 — the transfer never completes. Transfers work fine against a direct peer.

Root cause

For an in-dialog UAS request, the dialog's Record-Route set must be applied reversed (RFC 3261 §12.1.1 / §16.12.1.2). NewReferRequest/NewByeRequest already do that reversal when building the request — appending the response's Record-Routes in reverse order as the request's Record-Route set.

swapSrcDst then converts that set to Route headers with PrependHeader per entry — which reverses it a second time, restoring wire order. With a multi-hop proxy, the resulting Route set ends with our own Record-Route as the hop the proxy forwards to: the proxy strips its entry and bounces the REFER back to LiveKit-SIP → 405.

Fix

Append the Route entries in stored order (they are already correctly reversed for the UAS direction). Also fix TestRouteSet's UAS expectation to the RFC-correct reversed order (it was asserting the buggy wire-order).

• inbound REFER: Route set now [ours, proxy] — we route through the proxy once, it forwards to the peer
• inbound BYE: same conversion path, fixed together

Fixes #771

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.53%. Comparing base (0460b40) to head (b110feb).
⚠️ Report is 366 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #822      +/-   ##
==========================================
+ Coverage   65.25%   73.53%   +8.28%     
==========================================
  Files          51       43       -8     
  Lines        6588     7498     +910     
==========================================
+ Hits         4299     5514    +1215     
- Misses       1915     1984      +69     
+ Partials      374        0     -374     

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[rkfshakti]

The multi-proxy review finding is fixed on b110feb: inbound UAS BYE/REFER now builds its Route set from the original INVITE Record-Route headers in wire order, excludes the locally inserted Record-Route, and verifies both header order and transport destination. The focused route test passes repeatedly and all checks are green. A re-review would be appreciated; happy to make any further changes.