mypy: type-check tests and use pathlib fixtures

[DanielNoord]

Summary

• Remove the tests-specific mypy override and annotate test helpers and fixtures so the suite passes strict type checking.
• Replace legacy tmpdir/tmpdir_factory fixtures and path operations with typed tmp_path/tmp_path_factory and pathlib APIs.

Validation

• uv run mypy passes (86 source files).
• uv run python -m compileall -q isort tests passes.
• Ruff lint, Ruff formatting, and isort checks pass for all changed files.
• uv run pytest tests/unit: 621 passed, 1 skipped, 1 failed. The existing test_isort_can_turn_off_import_adds_with_action_comment_issue_1737 failure expects a typing.Any import absent from the actual output.

This pull request is intentionally kept as a draft.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.41%. Comparing base (1fd0701) to head (a64712a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2685      +/-   ##
==========================================
- Coverage   99.44%   99.41%   -0.04%     
==========================================
  Files          41       41              
  Lines        3231     3231              
  Branches      690      690              
==========================================
- Hits         3213     3212       -1     
- Misses         11       12       +1     
  Partials        7        7              

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