Avoid rewrapping styled trailing fills on resize

[dajiaohuang]

Summary

• Use the active background color and erase-to-end-of-line for safe, same-style background-only trailing fills, avoiding literal spaces that rewrap when the terminal narrows.
• Keep literal-space rendering for styles with other visual attributes or zero-width escapes, and skip erasure when the prior fill already matches.
• This covers the styled-fill cause reported in Terminal resize causes prompt_toolkit to print the prompt and the current input again and again #1933. The separate empty-row redraw cause overlaps Unwanted empty space at end of output #2008 and is addressed by Don't redraw a blank line just because its style differs #2088. This PR does not close Terminal resize causes prompt_toolkit to print the prompt and the current input again and again #1933.

Verification

• uv run pytest tests/test_renderer_trailing_fill.py: 3 passed.
• Ruff check/format, mypy, and git diff --check: passed.
• Full suite: 150 passed, 5 skipped, 4 failed. The same four failures reproduced on the base commit: two input-history assertions, the memory-leak check, and the Windows console-buffer test.
• Matched 120×24 benchmark: styled-fill median improved from 2.708 to 0.374 ms/call (7.24×); plain-row control was 0.156 to 0.154 ms/call, within sample variation.

Compatibility

The VT CSI K erase uses the active background only when the terminal supports and enables background-color erase (bce). A terminal with BCE disabled may erase to its default background; the issue report names GNU screen with defbce off as an example. See ncurses terminfo(5) and xterm control sequences. The implementation limits this path to background-only fills and preserves literal rendering for other styles.