[adamtheturtle]

Summary

• add HTTPX2Transport and AsyncHTTPX2Transport with native HTTPX2 configuration, clients, responses, and exception behavior
• preserve HTTPX as the default and ensure CoderPad clients close either built-in transport family
• exercise real synchronous and asynchronous HTTPX2 requests and document installation, selection, Screen usage, and migration

The tests use pytest-httpx2, whose dedicated httpcore2 mocker works with RESPX 0.23.1, so this change does not depend on lundberg/respx#317 or alias_httpx().

Validation

• uv run --extra=dev pytest -q --cov-fail-under=100 --cov=src/ --cov=tests . (259 passed, 100% coverage)
• all prek pre-commit, pre-push, and manual hooks

Fixes #385

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Note

Low Risk
Additive transport layer and dependency with HTTPX remaining default; risk is mainly install surface and callers who mix httpx/httpx2 config objects at transport boundaries.

Overview
Adds opt-in HTTPX2 as an alternate HTTP stack alongside the existing HTTPX default: new HTTPX2Transport and AsyncHTTPX2Transport wrap pooled httpx2 clients, accept only httpx2 limits/proxy/timeout objects, and map responses into the same TransportResponse type.

CoderPad and AsyncCoderPad treat HTTPX2 transports like HTTPX for lifecycle (close / aclose on the main transport and optional screen_transport). Default construction and client APIs are unchanged.

Packaging and docs: runtime dependency httpx2>=2.12.0, dev pytest-httpx2, README/Sphinx sections on migration and separate Screen transport; Sphinx nitpick ignores for httpx2 config types.

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