Fix atomic append to missing files

[harbinresearcher]

Describe your changes

Only read the previous contents during atomic append if the destination exists. This lets atomic append create a missing file, matching non-atomic append, while preserving the existing temporary-file replacement and Windows cleanup handling.

Add eight native-filesystem regression cases covering atomic/non-atomic append, UTF-8/UTF-16, empty/nonempty first writes, nested parent creation and a subsequent append. Against the unchanged source, four atomic cases fail and the four non-atomic cases pass.

Validation:

• Windows Python 3.12.14: baseline 302 passed/3 skipped; patched 310 passed/3 skipped. Both had one pytest cache permission warning.
• Windows Python 3.10.21: baseline 302 passed/3 skipped; patched 310 passed/3 skipped (one cache warning each).
• Linux Python 3.12.15: baseline 305 passed; patched 313 passed.
• Both full-suite comparisons reached 98.40% coverage (required minimum: 90%).
• Pinned pre-commit hooks, source mypy, sdist/wheel build and installed-wheel UTF-8/UTF-16 smoke checks passed.

This does not change concurrent append guarantees or the error/cleanup behavior discussed in #194. The reproduction uses real temporary files and no fault injection. AI assistance was used for investigation, tests, implementation and review; no independent human review is claimed.

Related issue

Closes #201

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests for the proposed changes.
• I have run the tests and there are not errors.

[harbinresearcher]

The Codacy check reports two Bandit B101 findings on the new pytest assertions in tests/test_io.py (lines 210 and 214): "Use of assert detected". Both are test assertions, following this module's existing pytest style; neither is a runtime security check. I have retained them rather than suppressing the rule or changing repository settings.

The full suite passes locally on Windows Python 3.10/3.12 and Linux Python 3.12. The GitHub Actions "Test package" run is currently action_required, awaiting maintainer approval to run this first fork contribution. Remote CI is therefore not yet verified.

Codacy findings: https://app.codacy.com/api/v3/analysis/organizations/gh/fabiocaccamo/repositories/python-fsutil/pull-requests/202/issues
Actions: https://github.com/fabiocaccamo/python-fsutil/actions/runs/38503952366