feat(tests): add pytest-codeblocks for documentation code snippet CI

[haoyu-haoyu]

Summary

Add automated testing infrastructure for documentation code examples using pytest-codeblocks, so code snippets stay in sync with the evolving codebase.

Changes

New dependency

• pytest-codeblocks>=0.17.0,<0.18 added to dev dependencies

CI integration

• New step in .github/workflows/ci.yml runs pytest --codeblocks on docs/quickstart.md, docs/cookbook/, and docs/tutorials/
• Runs on Python 3.12 only (pytest-codeblocks not verified for 3.13)
• Uses continue-on-error: true so snippets can be incrementally enabled

Skip markers

• 56 Python code blocks across 11 doc files marked with <!--pytest.mark.skip--> (these require healthchain package + external services)
• Files covered: quickstart, all 5 cookbooks, 6 tutorial pages

pytest configuration

• Added [tool.pytest.ini_options] with codeblocks marker in pyproject.toml

Usage

# Test all doc snippets
uv run pytest --codeblocks docs/quickstart.md docs/cookbook/ docs/tutorials/

# Test a specific file
uv run pytest --codeblocks docs/quickstart.md

To make a snippet testable, remove its <!--pytest.mark.skip--> marker and ensure it runs standalone.

Design decisions

• Skip-first approach: All existing snippets are skipped initially since they depend on the healthchain package, FHIR servers, spaCy models, etc. Snippets can be incrementally enabled as they are made self-contained.
• Scoped to curated docs: Only quickstart, cookbook, and tutorials are tested — reference/API docs are excluded to avoid executing auto-generated content.
• Single Python version: Runs on 3.12 only to avoid compatibility issues with the pytest-codeblocks package.

Closes #164