[dailyun]

What is the problem that this PR addresses?

Accepting / in the path prompt returns an empty string. It happens both when typing the root and when accepting it as the default. The trailing-separator cleanup removes the root separator along with ordinary directory separators; on Windows the same cleanup can turn a drive root into a drive-relative path.

How did you solve it?

Keep a path unchanged when it equals its native Path.anchor. Continue removing one trailing separator from non-root paths.

The input-pipe tests cover typed/default filesystem roots, separator-only roots, Windows UNC roots, and the existing non-root cleanup. Four root cases fail against the original code on macOS.

Validation on macOS arm64 / Python 3.13.15:

• Locked prompt_toolkit 3.0.48 / pytest 9.0.3 / pytest-cov 5.0.0: 175 passed, 1 skipped, 94% overall coverage.
• prompt_toolkit 3.0.53 was also tested: 175 passed, 1 skipped.
• prompt_toolkit 2.0.10: 167 passed, 9 skipped.
• Configured Black, isort, Flake8 and autoflake checks pass on both changed files; git diff --check passes.
• Mypy 2.1.0 with the locked prompt_toolkit 3.0.48: passes on all 19 source files. With the newer prompt_toolkit 3.0.53, four PromptSession argument-type errors occur identically on the branch and unmodified base 5a68c2d.

Native Windows Server 2025 / Python 3.13 verification also passes: 175 tests, no skips, including the UNC-root test, plus Mypy on all 19 source files. Fork CI run. The verification workflow is on a separate fork-only branch based on this exact PR commit; it is not part of this PR.

Developed and tested with OpenAI Codex assistance; no human review is claimed.

Checklist

• I have read the Contributor's Guide.
• I will check that all automated PR checks pass before the PR gets reviewed.