Fixes editor hit testing when the editor is clipped, most easily noticeable in a partially off-screen markdown block in the agent panel.

PositionMap::point_for_position was indexing into self.line_layouts relative to the editor's scroll position (which in this example doesn't apply because these editors don't scroll), rather than relative to self.visible_row_range.start. This caused the reported column to not always be computed using the correct row, which especially impacted rows closer to the bottom of the editor. Notice how closer to the bottom, point_for_position reports the correct row, but reports the column as 0:

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments
• The content is consistent with the UI/UX checklist
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable

Release Notes:

• Fixes selection not always matching the mouse position in a markdown block in the agent panel