feat(tests): add pytest conftest.py hierarchy with shared fixtures - Fixes #49
- Created tests/conftest.py with 12 shared fixtures (environment mocks, LangChain/ChromaDB mocking, configuration)
- Created tests/unit/conftest.py with 6 unit-specific fixtures (data vendors, sample data)
- Created tests/integration/conftest.py with 2 integration fixtures (live ChromaDB, temp dirs)
- Added pytest.ini with 7 custom markers (unit, integration, e2e, llm, chromadb, slow, requires_api_key)
- Added tests/test_conftest_hierarchy.py with 83 tests validating fixture infrastructure
- Updated docs/testing/README.md and writing-tests.md with fixture usage documentation
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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