Reset CVE v5 rank state before each import

[adulau]

Motivation

• Ensure the rank:cpe / vendor-product ranking produced from CVE v5 NDJSON does not accumulate across repeated imports and starts from a clean state each run.
• Centralize the rank-reset logic so it can be reused and tested instead of duplicating key deletion in the import script.
• Provide an opt-out for users who intentionally want cumulative behavior.

Description

• Add a reset_rank_state(rdb, missing_word_key) helper in lib/cpeimport/cvelistv5.py and export it via lib/cpeimport/__init__.py to delete rank:cpe, rank:vendor_product, and the configured missing-word set.
• Update bin/import_cvelistv5.py to call reset_rank_state() by default before importing and add a --preserve-rank flag to keep previous behavior when requested.
• Add a regression test test_reset_rank_state_clears_previous_import_data to tests/test_cvelistv5.py to verify the reset clears only the CVE v5 ranking state and does not remove unrelated w:<word> data.
• Update README.md to document the new default behavior and the --preserve-rank opt-out.

Testing

• Ran python -m unittest tests.test_cvelistv5 and the test suite passed (all tests OK).
• Ran python -m unittest tests.test_nvd_json and the test suite passed (all tests OK).

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Codex Task