feat(testing): configure pytest and create shared fixtures for OPEA (story 4.1)

Summary

• Configure pytest for the OPEA overlay (pytest.ini, pyproject.toml test deps)
• Create shared mock fixtures for ArangoDB, Redis, vLLM, TEI, and the vendored comps library
• Add 28 self-tests verifying all fixture shapes and factory overrides
• Add CLAUDE.md with venv/test/lint instructions for genie-ai-overlay/
• Pre-populate sys.modules for the Docker-vendored comps library to enable local testing

Test plan

• cd genie-ai-overlay && pip install -e .[test] — install test dependencies
• python -m pytest tests/ -v — 28 self-tests pass
• ruff check tests/ — zero lint errors
• ruff format --check tests/ — formatting passes
• Code review passed (0 patches, 2 deferred findings for future stories)