Add pytest infrastructure (pyproject.toml, conftest.py) and unit tests
for modules that previously had zero test coverage:
- test_schema_roundtrip.py: to_dict() serialization for all data classes
- test_reddit_enrich.py: URL parsing, thread data parsing, comment filtering
- test_reddit_sc.py: ScrapeCreators Reddit search (query expansion, subreddit discovery)
- test_instagram_sc.py: Instagram relevance scoring, tokenization, depth config
Includes fixtures/reddit_thread_sample.json for reddit_enrich tests.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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