ci: reuse shared Poetry bootstrap

[DiogoRibeiro7]

Reuse the shared Poetry setup action across CI jobs while keeping each job's existing Poetry install command and dependency groups unchanged.

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Summary by cubic

Replaces the per-job Python setup and manual Poetry pipx install with the shared setup-poetry action in the CI workflow. Each job keeps its existing Python version, dependency groups, and install command, so behavior is unchanged; only the bootstrap step is deduplicated.

^{Written for commit 247d17f. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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