Enforce 100% type coverage in CI with Pyrefly

[jorenham]

Summary

Following #828 (comment), this ensures that the public API type coverage is kept at 100% by running pyrefly coverage check in CI.

There should be no need to route this through tox, because pyrefly already supports uvx natively.

Pull Request Checklist

• I acknowledge this project's AI policy.
• This pull request is not from my main branch.
  • Consider granting push permissions to the PR branch, so maintainers can fix minor issues themselves without pestering you.
• There's tests for all new and changed code.
• New APIs are added to our typing tests in api.py.
• Updated documentation for changed code.
  • New functions/classes have to be added to docs/api.rst by hand.
  • Changed/added classes/methods/functions have appropriate versionadded, versionchanged, or deprecated directives.
    • The next version is the second number in the current release + 1. The first number represents the current year. So if the current version on PyPI is 26.1.0, the next version is gonna be 26.2.0. If the next version is the first in the new year, it'll be 27.1.0.
• Documentation in .rst and .md files is written using semantic newlines.
• Changes (and possible deprecations) are documented in the changelog.

[hynek]

There should be no need to route this through tox, because pyrefly already supports uvx natively.

That is true, but I try to have everything that needs to pass offline too with online/offline parity, such that pushing a passing tox has good chances to pass in CI too.

Just add it to the existing pyrefly tox env and call it a day – thanks!

[hynek]

sweet, thanks!