[HTTPXodus] migrate httpx to httpx2 with dual import

[ProgrammerPlus1998]

Closes #3929

🏷️ Part of HTTPXodus — a community effort to help major Python projects plan their path off the stalled httpx stable line onto httpx2, the actively maintained fork by Pydantic Services. One coordinated PR per project — no drive-by changes.

What this PR does

Switches FastChat's single httpx call site to httpx2 via a dual import. On Python ≥ 3.10 the runtime binds to httpx2; on 3.8 / 3.9 (which FastChat still supports and which httpx2 cannot run on) it falls back to httpx. requires-python is not changed — this PR does not drop any currently-supported interpreter.

Diff summary

2 files, +6 / −2 (commit 4ea6a03):

• pyproject.toml — added "httpx2>=2.12.0; python_version >= \"3.10\"" next to the existing "httpx". The python_version marker means httpx2 is only installed on runtimes that can run it; httpx stays in deps for 3.8/3.9.
• fastchat/serve/openai_api_server.py — top-of-file import httpx replaced with the try: import httpx2 as httpx; except ModuleNotFoundError: import httpx block. This is the only call site in the entire package; non-streaming paths use aiohttp, and controller / model-worker / gradio / monitor / judge code never imports httpx.

Test results

Validated in a fresh venv (Python 3.12.12):

• pip install -e . — resolves to both httpx-0.28.1 and httpx2-2.12.0 (the marker correctly selects httpx2 on 3.12).
• srv = importlib.import_module("fastchat.serve.openai_api_server"); srv.httpx.__version__ == "2.12.0" — the try branch wins.
• Fallback test: uninstalled httpx2, re-imported the module → srv.httpx.__version__ == "0.28.1". The except branch works as expected. This simulates a 3.8/3.9 install where httpx2 is not present.
• python -W error -c "import fastchat.serve.openai_api_server" — no import-time warnings.
• black==23.3.0 --check fastchat/serve/openai_api_server.py — clean (the project pins black==23.3.0 in its dev extras; format.sh uses the same version).

The repo's tests/test_image_utils.py fails to collect on stock main (pre-existing — it imports resize_image_and_return_image_in_bytes and image_moderation_filter from fastchat.utils, which are not defined there). Verified by stash + re-test on a clean checkout. Unrelated to this PR. The remaining tests/test_*.py files are all integration tests requiring a live model worker + controller + api server, which can't run in a unit-test context. None reference httpx directly.

Notes for reviewer

• This is one of the smaller migrations in the HTTPXodus batch: a single internal call site with no public-API surface change. The blast radius is the streaming-completion path between the OpenAI-compatible API server and a model worker.
• ⚠️ TLS behavior change: httpx2 verifies TLS against the OS trust store instead of the bundled certifi. FastChat deployments that front worker endpoints with TLS (e.g. reverse-proxied multi-node setups) and rely on a custom certifi CA bundle may need SSL_CERT_FILE / SSL_CERT_DIR after the switch. Worth a line in the deployment docs.
• FastChat's own development has slowed (last release v0.2.36 in 2024-02, last commit on main is a constants.py bump in 2025-06-02; the hosted Chatbot Arena moved to LMArena in 2024-09). The migration itself is tiny and safe; whether to take it is a judgement call about how much ongoing investment this stack is getting. We're flagging it because httpx is unpinned in pyproject.toml, so a breaking httpx 1.0 stable would land in installs without warning.
• requires-python is unchanged at >=3.8. If maintainers later decide to drop 3.8/3.9, the dual-import can be replaced with a hard import httpx2 in a follow-up PR.

Happy to revise per review — and equally happy to close this PR if the maintainers would rather wait for httpx 1.0 stable. 🙏