Don't highlight partial indent guide backgrounds (#34433)

Closes https://github.com/zed-industries/zed/issues/33665

Previously if a line was indented something that was not a multiple of
`tab_size` with `"ident_guides": { "background_coloring": "indent_aware"
} }` the background of characters would be highlighted. E.g. indent of 6
with tab_size 4.

| Before / After |
| - |
| <img width="497" height="77" alt="Screenshot 2025-07-14 at 14 43 46"
src="https://github.com/user-attachments/assets/93923117-047d-4d21-9a4f-488345f1ab89"
/>
| <img width="481" height="84" alt="Screenshot 2025-07-14 at 14 43 09"
src="https://github.com/user-attachments/assets/a5d383cb-50c3-4239-ae8c-f72765ae7287"
/> |

CC: @bennetbo Any idea why this partial indent was enabled in your
initial implementation
[here](https://github.com/zed-industries/zed/pull/11503/files#diff-1781b7848dd9630f3c4f62df322c08af9a2de74af736e7eba031ebaeb4a0e2f4R3156-R3160)?
This looks to be intentional.

Release Notes:

- N/A

crates/multi_buffer/src/multi_buffer.rs

@@ -5905,7 +5905,6 @@ impl MultiBufferSnapshot {
 
             let depth = if found_indent {
                 line_indent.len(tab_size) / tab_size
-                    + ((line_indent.len(tab_size) % tab_size) > 0) as u32
             } else {
                 0
             };