Preserve cursor position when pasting an empty clipboard in Vi mode

[FanWu-ai]

Pressing P with the default empty clipboard moves the cursor one character left even though no text was inserted. A following edit consequently acts on the wrong character: abcd, Escape, P, x produces abd instead of abc. Direct Document use can also return a negative cursor at the beginning.

Apply the paste-before cursor adjustment only when characterwise clipboard data is nonempty. Empty linewise data still inserts a blank line. Add direct document regressions and real PromptSession tests for P and 3P, with controls for other paste modes and linewise data.

Validation on Linux / Python 3.12.14:

• Pristine full suite: 156 passed.
• Six new regression cases fail against the original implementation; ten controls pass.
• Final full suite under coverage: 172 passed.
• Ruff lint/format, typos, strict mypy for Linux/Windows/macOS, all 12 pinned hooks, wheel/sdist builds, and git diff --check passed.
• Differential checks cover 42 corrected cases and 1,470 unaffected combinations; packaged source matches tested source.
• An independent 135-case empty-paste identity check passes.

Builds emit existing setuptools license-classifier deprecation warnings. Other Python versions and native operating systems were not run locally; mypy platform targets are static checks. Hosted CI has not run yet.

AI assistance was used to investigate the bug and prepare the implementation and regression tests.