[arcwiser]

What

Allow the overlap encoding regression fixture to mock subprocess._text_encoding on Python versions where that private helper does not exist. Python 3.10 otherwise fails all tests using command_output before exercising the UTF-8 handling they cover.

Tested

• Windows, Python 3.10.11: python -m unittest tools.tests.test_pr_overlap_encoding (10 passed)
• Windows: python tools/check_conventions.py --base origin/main
• Windows: git diff --check origin/main
• Confirmed no open issue or pull request covers this Python 3.10 test failure

Checklist

• Based on current main; no other open PR implements the same functions
• One topic per PR; follow-ups go in a new PR
• No comments in code except technical debt
• Unimplemented paths throw; no implementation paths changed
• No notes, investigation .md files or images added to the repository
• No third-party code added
• Generic behaviour, not specific to one title
• Depends on: none
• Unblocks: none
• AI-assisted: yes

[arcwiser]

Closing as a duplicate of #4361. The project overlap checker identified the shared test line after opening; #4361 has broader cross-version coverage and should remain the single implementation. Sorry for the duplication.