Clear scroll marks when clearing the buffer

[SushaanthSrinivasan]

Summary

Fixes #20086 — Ctrl+Shift+K (Clear Buffer) now clears scroll marks from the scrollbar.

References and Relevant Issues

• Closes Clear buffer doesn't clear scroll marks #20086

Detailed Description

ControlCore::ClearBuffer() sends VT escape sequences to clear the buffer. The scrollback path (\x1b[3J) already clears marks via TextBuffer::ClearScrollback, but the screen-clearing path doesn't touch marks — they persist stale in the scrollbar.

This adds a ClearAllMarks() call inside the existing lock block after the VT sequences are written, for both Screen and All clear types.

Validation Steps

1. Enable "showMarksOnScrollbar": true in profile defaults
2. Open a PowerShell tab, hit Enter ~20 times to generate scroll marks
3. Press Ctrl+Shift+K — marks should disappear
4. Hit Enter again — new marks should appear normally

before.after.mp4

• Closes Clear buffer doesn't clear scroll marks #20086
• Tests added/passed
• Documentation updated
• Schema updated

[DHowett]

I am surprised that the Erase in Screen handler does not already do this.

It seems to me that fixing the issue in this layer will not fix the case in issue #20086 with the clear command, because the clear command does not run through this code.

We should fix this in the appropriate layer if possible.

[SushaanthSrinivasan]

added ClearMarksInRange to _EraseAll() in adaptDispatch.cpp — fixes clear/cls which go through the \x1b[2J path.

ctrl+shift+k still needs the ControlCore fix though — ClearBuffer() uses DL + ED0 to preserve the cursor row (GH#18732), so it never hits _EraseAll.

pushed both fixes, tested all three paths (ctrl+shift+k, bash clear, powershell cls). dropping the screen rec below:

wt.scroll.clear.mp4