fix Upward Find for venv #759

[asukaminato0721]

Summary

Fixes #759

keeping the existing per-root scan and applying it across project_path plus its ancestors, so Pyrefly now finds the nearest ancestor .venv instead of only searching downward from the current root.

Test Plan

add test

[Copilot]

Pull request overview

Updates Pyrefly’s virtual environment discovery to search upward through ancestor directories so a project can use a .venv located in a parent directory (addresses #759).

Changes:

• Refactors venv discovery into a per-root helper (find_in_root) and applies it across project_path ancestors.
• Adds unit tests ensuring ancestor search works and prefers the nearest matching ancestor venv.

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[connernilsen]

Hey @asukaminato0721, thanks for doing this. This has been on the todo list for a while, so it's awesome that you picked it up.

There are a few requests below, but I don't think they should be too difficult to fix. Let me know if you have further questions or want to discuss anything.

[connernilsen]

Two more minor things, then I'll pull it in and merge it :)

[connernilsen]

Thanks for working on this!

[meta-codesync]

@connernilsen has imported this pull request. If you are a Meta employee, you can view this in D98169929.