fix(sip_client): keep the proxy path in responses and ACKs

[eigger]

Fixes #323

Summary

Calls through a proxy/SBC (reported with 3CX) could not complete because sip_client did not preserve the proxy path in responses and ACKs:

• The 200 OK to an inbound INVITE echoed only the top Via and dropped Record-Route, so the proxy's ACK never reached us and the call stayed in ANSWERING.
• The ACK for a 407 used a fresh Via branch, so the PBX kept retransmitting the 407 while the authenticated INVITE was already in flight.

This ports the equivalent hass-sip fixes (eigger/hass-sip#22, eigger/hass-sip#38, eigger/hass-sip#40) rather than applying the patch attached to the issue verbatim, so the same routing rules are used in both projects.

Changes

• parse_sip_message: Via / Record-Route / Route / Service-Route rows are joined in wire order instead of keeping only the first occurrence; header folding is supported. New helpers split_header_values, via_branch, is_loose_route, extract_angle_uri, apply_route_set.
• build_response_: echoes every Via; adds Record-Route + Contact to every dialog-establishing INVITE response (18x and 2xx), RFC 3261 §12.1.1.
• build_ack_: a non-2xx ACK reuses the response's top Via branch and the INVITE Request-URI (§17.1.1.3), no Route; a 2xx ACK gets a new branch and goes through the route set of that response (so a forked 2xx is acknowledged along its own path).
• Route set: learned from Record-Route (UAS as-is, UAC reversed) and applied to in-dialog requests (BYE) with loose/strict router handling, §12.2.1.1, one Route: header per hop. CANCEL keeps the INVITE Request-URI (d_invite_uri_).
• is_loose_route: only ;lr after @ and before ? counts — <sip:user;lr=on@proxy> is strict. ;lr outside <> is a header parameter, not a URI parameter, and stays strict; misclassifying a loose router as strict is the safe direction (§16.4 strict-router compatibility).
• Tests: tests/native/sip_sdp/test_sip_message.cpp covers header parsing (multi-Via order, Record-Route rows vs comma list, nested commas, folding), via_branch, is_loose_route (incl. userinfo/headers-part lr), and apply_route_set (empty / loose / strict / reversed-2xx).

Differences from the patch in #323

• Record-Route/Contact are added only to 101–299 INVITE responses (the attached patch also put Record-Route on 100 Trying, and 18x responses still had no Contact).
• In-dialog requests (BYE) now carry Route headers built from the route set; the attached patch did not cover this, which would still break hangup through a proxy that is not a B2BUA.
• Multi-value headers are handled in the parser instead of re-scanning the raw message.

Known limitation (unchanged)

send_raw_ always sends on the socket connected to the configured server_. If the first Route hop is a different address, BYE / 2xx ACK still go to server_ — correct for a single SBC / outbound-proxy setup, which is what this component targets.

Test plan

• Native tests (test_sip_message.cpp, test_parse_sdp.cpp) pass with /W4 /WX (MSVC); run.sh runs in CI
• esphome compile tests/components/sip_client/test.esp32-idf.yaml builds with no warnings (xtensa-gcc 14.2)
• Reporter re-test against 3CX SBC: inbound answer → ACK received → IN_CALL; outbound 407 no longer retransmitted; BYE from both sides

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