saithriftv2: link VPP libraries for platform=vpp + bookworm/py3 build fixes

[nicholasching]

Context / motivation

This PR is part of the SAIVPP unit-test framework: a Docker harness (docker-sai-test-vpp) that runs the upstream OCP sai_test PTF suite against the real VPP SAI backend (libsaivs) in one container — VPP + saiserver + PTF + veth/AF_PACKET topology. That work is documented in our devdocs under sonic-sairedis/.azure-pipelines/docker-sai-test-vpp/devdocs/ (see progress.md).

To run the suite, saiserver (saithriftv2) must build and link against the VPP SAI backend on a bookworm/python3 toolchain. Upstream saithriftv2 only has a vs link line and a python2.7 install path, so it does not build for the vpp platform on bookworm. This is Phase 1, Task 2 of the SAIVPP UT HLD (the saithriftv2 Makefile work).

What this change does

Build/packaging only, no test logic:

• test/saithriftv2/Makefile — add a platform=vpp link line that pulls in the 5 VPP libraries SaiVppXlate.c requires (-lvlib -lvlibapi -lvppapiclient -lvlibmemoryclient -lvppinfra) plus -lswsscommon; also add -lswsscommon to the vs line.
• test/saithriftv2/src/saiserver.cpp — enable SWSS debug logging at startup (swss::Logger to STDOUT), so the harness captures a high-level SAI RPC trace in saiserver.log (used throughout our debugging).
• debian/python-saithrift.install — install the Python bindings from the python3 dist-packages path so the package builds on bookworm (was hardcoded to python2.7/site-packages).
• test/saithriftv2/convert_header.py — fix an enum __str__ that did split(".")[1] and threw an IndexError on some enum names; use self.name instead.

Scope / risk

• 4 files, +11/−3 — build flags, a packaging path, a logging init, and a one-line stringification fix.
• No change to SAI semantics, RPC surface, or test logic.
• The vpp link line is new (additive); the vs line only gains -lswsscommon.

[nicholasching]

@tjchadaga please run azure pipeline when possible; this PR is needed for an upstream feature at Cisco ASAP, thanks!

[yue-fred-gao]

This adds extra dependency on SONiC swss. This is not right for the projects running independently from SONiC.

[nicholasching]

I agree! I was using that before to route saiserver logs to stdout for debugging. We do not need it anymore; I have pushed another commit to remove it. Thanks Fred!