Replace `httpx` with `httpx2`

Thanks for stopping by to let us know something could be better!

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**Is your feature request related to a problem? Please describe.**
`httpx`'s Issues, PRs, and Discussions are now restricted to collaborators only — no new submissions are being accepted. Moreover, no stable release has shipped since `0.28.1` (Dec 2024).

**Describe the solution you'd like**
Replace `httpx` with [`httpx2`](https://github.com/pydantic/httpx2), an active fork by Pydantic.

**Describe alternatives you've considered**
Keep `httpx` as-is for compatibility.

**Additional context**
[Closing announcement from the httpx maintainer](https://github.com/encode/httpx/discussions/3784)