## Context

Fixes the visual selection update in `TextDiffView::open` to properly
convert between buffer-local and multibuffer coordinates using
`buffer_point_to_anchor`. Previously, raw multibuffer Points were used
directly for line expansion, which produced incorrect regions when
expanded deleted diff hunks shifted multibuffer row numbers.

This provides a single unified code path for both singleton and
non-singleton multibuffers, as suggested in [#51457 review
feedback](https://github.com/zed-industries/zed/pull/51457#issuecomment-4091134303).

Follow-up to #51457.

## How to Review

Small PR - all changes are in `crates/git_ui/src/text_diff_view.rs`.
Focus on:

- `open()`: The visual selection update block now uses
`buffer_point_to_anchor` + `to_point` instead of raw multibuffer
coordinate math
- No more assumption that multibuffer Points == buffer-local Points
- Existing tests validate both singleton and non-singleton multibuffer
paths

## Self-Review Checklist
- [x] I've reviewed my own diff for quality, security, and reliability
- [x] The content is consistent with the UI/UX checklist
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

Release Notes:

- Fixed incorrect diff region when using "Diff Clipboard with Selection"
with expanded diff hunks in the editor.