feat(packages): python client (openchainbench) for PyPI

[Flotapponnier]

What

Adds a Python client package under packages/python-client/ that wraps the
public OpenChainBench API: /api/citable, /api/stat/, /api/series/.

Why

Distribution channel for the citable API. Python devs and AI agents expect
pip install openchainbench rather than DIY HTTP. Bonus: pypi.org/project/openchainbench
sidebar surfaces 4 to 5 dofollow project links to openchainbench.com (Homepage,
Documentation, Repository, Issues, Changelog).

Shape

• Synchronous client on httpx, context-manager friendly, typed exception
  hierarchy (NotFoundError, RateLimitError, APIUnavailableError) so callers
  branch on intent not HTTP status.
• Models are frozen dataclasses. Parsing is permissive on extras and tolerant
  of payload polymorphism (source can be str or object, methodology can be
  str or list).
• 15 tests: 12 mocked via httpx.MockTransport, 3 hit live openchainbench.com
  (skip with OCB_SKIP_INTEGRATION=1 for offline CI).
• python-client-ci.yml runs unit tests on Python 3.10, 3.11, 3.12, 3.13 +
  builds sdist + wheel on every PR touching the package.

Publish flow

• Tag push python-v* triggers .github/workflows/pypi-publish.yml.
• Workflow builds, smoke-tests the wheel on 3.10-3.13, then publishes via
  PyPI Trusted Publisher (OIDC, no token stored in the repo).
• Namespaced python-v* so existing OCB site tags (v1.0-dataset, etc.) never
  trigger a Python release.

One-time setup required from the user before first release

Documented in packages/python-client/PUBLISHING.md. TL;DR:

1. Create a PyPI account at https://pypi.org.
2. Add a pending publisher at https://pypi.org/manage/account/publishing/ with:
   • Project: openchainbench
   • Owner: ChainBench
   • Repo: OpenChainBench
   • Workflow: pypi-publish.yml
   • Environment: pypi
3. In GitHub repo Settings > Environments, create the pypi environment.
4. Tag and push: git tag python-v0.1.0 && git push origin python-v0.1.0.

Test plan

• pytest tests -m "not integration" green locally (12 passed)
• pytest tests green against live API (15 passed)
• python -m build produces wheel + sdist
• wheel contains only openchainbench/ + dist-info + LICENSE + py.typed
• After merge, user configures PyPI Trusted Publisher (PUBLISHING.md)
• First tag python-v0.1.0 -> CI publishes to pypi.org/project/openchainbench/