Migrate httpx to httpx2 - #16358

What?

• Swaps the test-client dependency in the standard, standard-no-fastapi-cloud-cli and all extras from httpx >=0.23.0,<1.0.0 → httpx2 >=2.0.0, same for the github-actions dependency group. fastapi.testclient itself needs no change — Starlette's TestClient already prefers httpx2 and falls back to httpx (Support httpx2 in the test client Kludex/starlette#3291).
• Moves the repo's own httpx usage to httpx2: scripts/sponsors.py, scripts/notify_translations.py, the scripts/playwright screenshot helpers, and the docs_src/async_tests example.
• Updates the docs (English plus all 12 translations) and README.md to name httpx2 and point at https://pydantic.dev/docs/httpx2/.
• Keeps httpx in the tests group, solely so the existing without-httpx2 CI job still has a client to fall back to.

Why?

• related: Swap from httpx to httpx2 #16186
• httpx is unmaintained. httpx2 is the maintained fork of 0.28.1 with the same public API, and Starlette's TestClient already prefers it — so fastapi[standard] currently installs the deprecated stack and only avoids the deprecation warning by accident of resolution order.

Additional notes

• The ## HTTPX heading in advanced/async-tests.md becomes ## HTTPX2, and the add-permalinks-pages --update-existing hook regenerates anchors from titles, so #httpx → #httpx2. External deep links to that section break; I don't see a way to keep the old anchor without fighting the hook.
• docs_src/async_tests/app_a_py310/test_main.py now imports httpx2 directly, which is exactly what the without-httpx2 job uninstalls. I gated its wrapper in tests/ with pytest.importorskip rather than put a conditional import into example code that users read.
• Translated prose got a mechanical rename. I fixed the Turkish suffixes that broke because httpx2 is vowel-final where httpx wasn't (HTTPX2'yi, HTTPX2'nin); the other languages may still want a native reviewer's eye.
• Full suite passes both ways: 3348 passed with httpx2, and 3344 passed / 18 skipped with it uninstalled.
• This touches pyproject.toml and uv.lock, so guard-dependencies will likely auto-close it — happy to move the proposal back into Swap from httpx to httpx2 #16186 if that's preferred.
• Generated-by: Claude Code (Opus 5). I pointed it at the repo's httpx usage; it did the dependency, source and docs changes and verified both CI paths. I reviewed the diff and wrote this description.