Use buffer line numbers when copying file locations

[apoorva-01]

Objective

Fixes #54250

editor: copy file location copies the row you're on in the multi-buffer, not the row in the actual file. In a git diff view with collapsed lines it copies the wrong line number.

Solution

It read the rows straight off the selection, which is in multi-buffer coordinates. ClipboardSelection::for_buffer already handles this with range_to_buffer_range, so I used the same thing. Path and line now come from the same buffer.

range_to_buffer_range returns nothing when the selection ends in a different buffer than it starts in, which you can do in a git diff view. Rather than copy nothing, it falls back to the cursor's buffer.

Testing

Three tests: a multi-buffer excerpt, a selection dragged across two files, and a plain single-file editor. The multi-buffer ones fail on main; the singleton one passes either way, so normal editors are unaffected.

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

Release Notes:

• Fixed editor: copy file location copying the wrong line number in multibuffers (#54250).