Hide placeholders after a prompt finishes

[x0Lazarus]

Pressing Enter on an empty prompt leaves its placeholder on the completed line, so earlier prompts can look as though they are still waiting for input. Hide the placeholder during the final redraw, using the same is_done filter as other temporary prompt elements. It remains visible while the prompt is active, and submitted text is unchanged.

The regression tests inspect the rendered screens for plain and ANSI-formatted placeholders, with both empty and nonempty input. All four pass; the two empty-input cases fail before the change.

On Windows with Python 3.12, the full suite has 151 passing tests, five existing Windows skips and four failures also reproduced against unchanged source: two history tests and two tests requiring a Windows console. Ruff check/format, Typos and strict Mypy checks for all three configured platforms pass. Other operating systems and Python versions were not tested locally.

Fixes #1997.