test(backend): cover the live reader-values CSV path

[lewisjared]

Adds direct tests for the code paths the audit found untested: the reader-backed CSV exports, chunked file streaming, and the config secret guard.

• test_reader_values.py pins the CSV column contract using real ScalarValueCollection/SeriesValueCollection instances, covering header layout with and without outlier detection, the X-REF-* headers, NaN sanitization, and the series index fallbacks.
• test_file_handling.py covers file_iterator chunking.
• test_config.py covers the default-secret guard (raises outside local, warns in local) and parse_cors.

These pin the CSV contract that previously was only asserted against a dead pre-Reader copy of the generators (removed in #42). A future climate-ref bump that changes the collection shapes will fail here first.

[netlify]

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Configuration used: Organization UI

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Run ID: 30b05ec1-272a-4837-84e4-eb3d479342bc

📥 Commits

Reviewing files that changed from the base of the PR and between ddf8749 and a57a7f3.

📒 Files selected for processing (4)

• backend/tests/test_core/test_config.py
• backend/tests/test_core/test_file_handling.py
• backend/tests/test_core/test_reader_values.py
• changelog/43.trivial.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch test/reader-values-coverage

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