fix: keep short MarkdownStream updates fully visible

[Alphaxiaoteng]

Summary

• Fixes BUG: MarkdownStream can drop leading lines when live output is shorter than live_window #5654: when rendered output is shorter than live_window, num_lines - live_window becomes negative and lines[num_lines:] keeps only a suffix (e.g. 4 lines with window 5 → only the last line).
• Clamp with max(0, num_lines - live_window) so all early lines stay in the Rich live area until they are actually stable.
• Add regressions for the short-window and longer-than-window cases.

Test plan

• pytest tests/basic/test_mdstream_short_window.py -q → 2 passed
• Short case (live_window=5, 4 lines): all four lines retained in Live
• Longer case (live_window=6, 8 lines): first two printed above Live, last six retained

Made with Cursor

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