docs: add httpx2 as challenger to HTTP Clients

Pydantic Services took over stewardship of the stalling httpx under the httpx2 name, Starlette already switched its TestClient, and it hit 144M downloads/month (pepy) within 3 months of first release at Production/Stable v2.12.0. Placed last in the challenger tier behind urllib3 per the downloads-descending ordering rule. Fork format and a rewritten description distinguish it from httpx, whose PyPI summary is identical.

Co-Authored-By: Claude <noreply@anthropic.com>

Author: vinta

README.md

@@ -366,6 +366,7 @@ _Libraries for working with HTTP._
   - [httpx](https://github.com/encode/httpx) - A next generation HTTP client for Python.
   - [aiohttp](https://github.com/aio-libs/aiohttp) - Asynchronous HTTP client/server framework for asyncio and Python.
   - [urllib3](https://github.com/urllib3/urllib3) - A HTTP library with thread-safe connection pooling, file post support, sanity friendly.
+  - [httpx2](https://github.com/pydantic/httpx2) - HTTP/1.1 and HTTP/2 client with sync and async APIs, maintained by Pydantic ([httpx](https://github.com/encode/httpx) fork).
 - URL Manipulation
   - [yarl](https://github.com/aio-libs/yarl) - Yet another URL library.
   - [httpx.URL](https://www.python-httpx.org/api/) - The immutable URL class bundled with HTTPX.