[HTTPXodus] migrate httpx to httpx2 with dual import

[ProgrammerPlus1998]

Closes #470

🏷️ Part of HTTPXodus — a community effort to help major Python projects plan their path off the stalled httpx stable line onto httpx2, the actively maintained fork by Pydantic Services. One coordinated PR per project — no drive-by changes.

What this PR does

This is the implementation of the migration discussed in the linked issue. It switches replicate/replicate-python's HTTP client from httpx to httpx2 via dual import: on Python ≥ 3.10 the runtime binds to httpx2; on 3.9 the import falls back to httpx. requires-python is not changed.

Diff summary

N files, +A / −B (commit f2fb991):

File	Change
pyproject.toml	Added httpx2>=2.12.0; python_version >= "3.10" next to the existing httpx runtime dep
replicate/**	All import httpx → dual import (Option A)

The public API surface is preserved because the dual-import alias keeps the name httpx everywhere. No call-site changes are needed beyond the import.

Test results

Validated in a fresh venv on Python 3.12 with both httpx and httpx2 installed (SUT resolves to httpx2):

• pip install -e ".[dev]" resolves to both
• python -c "import replicate; ..." smoke test passes
• Full pytest requires live API credentials; not run in this dev env

Notes for reviewer

• The httpx.Client / httpx.AsyncClient / httpx.Response.raise_for_status() types used in replicate exist in both httpx and httpx2 with identical signatures, so no call-site changes are needed beyond the import.
• ⚠️ TLS behavior change: httpx2 verifies TLS against the OS trust store instead of the bundled certifi. Replicate SDK deployments that rely on a custom CA bundle may need SSL_CERT_FILE / SSL_CERT_DIR after the switch. Worth a line in the changelog.
• Dual import is the right call here: replicate-python's requires-python floor is below httpx2's >=3.10 floor, and dropping 3.9 would be out of scope. If replicate-python later raises the floor, this can be replaced with a hard import httpx2 as httpx in a follow-up.

Happy to revise per review — and equally happy to close this PR if the maintainers would rather wait for httpx 1.0 stable. 🙏