Closes #47846

`visible_excerpts` computed the visible buffer range by adding display
line count directly to the buffer start row:

```rust
// Before
multi_buffer_visible_start + Point::new(visible_line_count, 0)
```
This ignores folds entirely. When a 700-line function is folded into one
display line, content after the fold is visible on screen but falls
outside the computed buffer range, so its brackets are never colorized.

The fix converts through display coordinates so the fold/wrap layers are
respected:

```rust
// After
let display_end = DisplayPoint::new(display_start.row + visible_line_count, 0);
let multi_buffer_visible_end = display_end.to_point(&display_snapshot);
```

### Results

**Before Fix**
<img width="852" height="778" alt="스크린샷 2026-03-10 오후 8 29 10"
src="https://github.com/user-attachments/assets/a0d2d81f-a8b2-4cf4-b1f3-cf5f8288a696"
/>

**After Fix**
<img width="1031" height="794" alt="스크린샷 2026-03-10 오후 8 32 27"
src="https://github.com/user-attachments/assets/2b0496b1-8302-4248-b73a-c31f5d0b0c4b"
/>

Before you mark this PR as ready for review, make sure that you have:
- [X] Added a solid test coverage and/or screenshots from doing manual
testing
- [ ] Done a self-review taking into account security and performance
aspects
- [ ] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)

Release Notes:

- Fixed bracket colorization not working for content after folded
regions and for functions with large bodies.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>