[cig13zs]

Builds on #317 and follows the direction from #316: one respx package that works with either httpx or httpx2.

• New respx/httpxlib.py, like httpx's jsonlib. Every respx module imports httpx and httpcore from there. It picks httpx if installed, otherwise httpx2. RESPX_HTTPX=httpx or RESPX_HTTPX=httpx2 picks explicitly, e.g. when both are installed. It has to be set before pytest starts, since the plugin imports respx early.
• The httpx mocker patches the picked client. The default httpcore mocker keeps patching both httpcore and httpcore2, as in Support httpx2 via httpcore2 #317, so requests can't slip through to the network when both are installed.
• Typing: type checkers assume httpx. Defining RESPX_HTTPX2 as always true (mypy always_true, pyright defineConstant) type-checks against httpx2 instead.
• Packaging: adds respx[httpx] and respx[httpx2] extras. httpx stays a regular dependency for now, so nothing changes for current installs.
• Tests run against httpx only, httpx2 only, and both installed (with each pick), via a tool parameter in the noxfile and the CI matrix. Coverage stays at 100% in each, and mypy runs for both.
• Short docs section under Installation.

I also ran openai-python's test suite with its vendored tests/respx2 pointed at this branch through a small adapter, and no test changed outcome.

Open question: with httpx still required, pip install respx[httpx2] also installs httpx, so httpx2 users need RESPX_HTTPX=httpx2 until httpx becomes optional. Dropping it from install_requires is a one-line change and the httpx2-only env already covers that layout. Happy to do it here or in a follow-up, whichever you prefer.

CI grows from 7 to 17 test jobs. If that's too many I can trim the matrix to fewer Python versions for the httpx2 and both-installed envs.