Fix adding breakpoint/bookmark via gutter context menu using the wrong row

[guopenghui]

Objective

Fixes #59265

Solution

Use display_point_to_anchor to properly convert DisplayRow to Anchor, instead of manually constructing Point directly as done currently.

Testing

• Tested manually, see video in issue
• Tested on windows

Self-Review Checklist:

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

Showcase

Click to view showcase

My super cool demos here

---

Release Notes:

• Fixed the issue where right-clicking to add breakpoints or bookmarks in multibuffer would place them on the wrong lines.