Add Responses API quickstart

[georgeatparallel]

What changed

• add a Python quickstart for cited research through the OpenAI-compatible Responses API
• make exactly one API call per invocation, then parse and deduplicate URL citation annotations
• surface a clear contract error when the response contains no citations instead of retrying client-side
• add deterministic tests, an opt-in single-call live test, a locked uv environment, and cookbook registration

Why

The public cookbook did not have a direct Responses API example. This gives developers familiar with the OpenAI SDK a small, copyable first run that needs only a Parallel API key.

The example intentionally keeps the product contract visible: one request either returns cited output or fails clearly. It does not add retry behavior that could create extra billed calls or conceal missing citation annotations.

Validation

• uv lock --check
• uv run pytest -m "not live" -q (7 passed, 1 deselected)
• RUN_LIVE_TESTS=1 uv run pytest -m live -q (1 passed, 7 deselected; exactly one API call)
• Ruff format and lint
• pre-commit secret and hygiene hooks
• cookbook JSON schema validation