chore: migrate HTTP client dependency from httpx to httpx2

[frostming]

Summary

Migrate the Streamable HTTP transport from httpx to httpx2.

httpx2 keeps the same public surface for everything this SDK touches —
AsyncClient, ASGITransport, MockTransport, Request, Response,
AsyncByteStream, Timeout — so the transport logic is unchanged and this is
essentially a module-name + dependency swap in both pyproject.toml and the
code.

Changes:

• pyproject.toml: httpx[http2]>=0.27 → httpx2[http2]>=2.12 in both the
  dev group and the http extra.
• src/acp/http/client.py: import switched to httpx2; type annotations and
  runtime calls (AsyncClient, Response, HTTPError, Timeout) updated.
• src/acp/_cookies.py, src/acp/http/__init__.py: docstring references.
• tests/http/*: switched to httpx2.
• docs/web-transport.md: dependency note for the [http] extra.
• uv.lock: drop httpx 0.28.1 + httpcore 1.0.9; add httpx2 2.12.0 +
  httpcore2 2.12.0 (+ truststore).

Note: starlette.testclient prefers httpx2 and only falls back to httpx
with a StarletteDeprecationWarning, so this also removes that warning path.

Related issues

Testing

Ran the full local gauntlet on this branch:

$ uv lock --locked
All checks passed!

$ uv run prek run -a          # ruff check + ruff format + hygiene hooks
ruff check .............................................................. Passed
ruff format ............................................................. Passed

$ uv run ty check --exclude "src/acp/meta.py" --exclude "src/acp/schema.py" \
    --exclude "src/acp/experimental/v2/meta.py" \
    --exclude "src/acp/experimental/v2/schema.py" --exclude "examples/*.py"
All checks passed!

$ uv run deptry src
Success! No dependency issues found.

$ uv run python -m pytest --doctest-modules
256 passed, 1 skipped in 9.45s

The HTTP/WS transport suites (tests/http/test_asgi.py, test_http_client.py,
test_fixes.py, test_loopback.py, test_cookies.py, test_sse.py,
test_http_server.py, test_websocket.py, test_ws_cookies.py) all pass,
confirming SSE streaming, cookie-jar session affinity, and the in-memory
MockTransport/ASGITransport paths work unchanged on httpx2.

Docs & screenshots

docs/web-transport.md updated to document that the [http] extra now pulls in
httpx2[http2]. No UX change, so no screenshots.

Checklist

• Conventional Commit title (chore:).
• Tests cover the change (existing HTTP/WS suites exercise the swapped
  client surface end-to-end).
• Docs/examples updated when behaviour is user-facing.
• Schema regenerations (make gen-all) are called out if applicable
  (not applicable — no schema changes).