Fix Poetry setup on Python 3.9

[michaelfeil]

Python 3.9 CI fails before linting because pipx exposes the runner's shared pip to the Poetry environment. That pip uses dataclass(slots=True), which Python 3.9 cannot execute. The failure is visible in the setup logs.

Install the requested Poetry version in a fresh virtual environment created by the selected Python, using that environment's own pip. Restore the existing dependency caches before installation, and replace the cached pipx environment and its hardcoded symlink repair with a portable setup. Expose only the Poetry launcher on PATH so subsequent python and pip commands keep using the selected project interpreter.

Add a lightweight setup workflow for Python 3.9 and 3.12 on Ubuntu and Windows. It verifies the Poetry version, checks that setup does not shadow Python, and installs/runs a dependency-free project.

Validation

• Executed the action's installation step and workflow's smoke-test step locally on Python 3.9.25 and 3.12.15, twice each, using temporary paths containing spaces. All four runs passed.
• actionlint 1.7.7 passes for the new workflow and existing test/lint/CI callers (optional shellcheck/pyflakes integrations disabled).
• git diff --check passes.
• Windows execution is covered by the new GitHub Actions matrix and awaits CI.

Related issue

Addresses the pre-existing setup failure observed in #675. This PR is based directly on main and contains only the setup fix and its smoke-test workflow.

[chatgpt-codex-connector]

Codex Review Summary

This comment shows the latest Codex review activity on this pull request.

Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-10T02:41:28.164482Z	ae65d7a	PR opened

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[greptile-apps]

Confidence Score: 5/5

[High impact] The PR appears safe to merge; no blocking issues were found.

Summary

This PR installs Poetry in a fresh environment created by the selected Python, replacing pipx and its hardcoded path repair.

• Poetry runs in its own environment without replacing the project Python.
• A setup check runs on Python 3.9 and 3.12 on Ubuntu and Windows.

_{Reviews (1) · Last reviewed commit: "Isolate Poetry installation from pipx sh..." · Reviewed by Greptile}

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.00%. Comparing base (1eb4396) to head (ae65d7a).
⚠️ Report is 1 commits behind head on main.
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #676      +/-   ##
==========================================
- Coverage   80.05%   80.00%   -0.06%     
==========================================
  Files          43       43              
  Lines        3485     3485              
==========================================
- Hits         2790     2788       -2     
- Misses        695      697       +2     

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