Keep trash cleanup non-fatal for locked files

[cyphercodes]

Summary

• Keep rmdir(..., safe_rm=False) non-fatal when files remain locked during trash cleanup.
• Preserve the existing manual-cleanup warning when the trash directory cannot be removed.
• Add regression coverage and a towncrier news fragment.

Fixes #1847

Test plan

• uvx --from 'tox>=4.45' tox run -e 3.11 -- tests/test_util.py -k rmdir
• uvx --from 'tox>=4.45' tox run -e 3.11 -- tests/test_util.py
• uvx --from 'tox>=4.45' tox run -e lint

Checklist

• ran the linter to address style issues (pre-commit run --all-files via tox run -e lint)
• wrote descriptive pull request text
• ensured there are test(s) validating the fix
• added news fragment in changelog.d/ folder
• updated/extended the documentation (not applicable beyond the news fragment for this internal cleanup fix)

[gaborbernat]

LGTM