Fix NOT_NOVEL response parsing

[justavibedev]

Summary

• normalize the prompted NOT_NOVEL token before classification
• preserve support for the legacy NOT NOVEL form
• add regression coverage for plain, explained, and Markdown-wrapped responses

Fixes #483.

Testing

• python -m unittest discover -s tests -p test_novelty_response_parsing.py -v
• black --check openevolve/database.py tests/test_novelty_response_parsing.py

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