ci: reuse shared Poetry setup for security

[DiogoRibeiro7]

Use the shared Poetry setup action in the dependency-audit job while preserving Python 3.12, Poetry 2.2.1, dependency export, strict pip-audit, and the separate CodeQL analysis.

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Summary by cubic

Reuses the shared Poetry setup action in the dependency-audit job instead of manually installing Python and Poetry, while keeping Python 3.12, Poetry 2.2.1, dependency export, strict pip-audit, and the separate CodeQL analysis unchanged.

^{Written for commit 44abf03. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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