gpui: Fix partial dashed border rendering (#38190)

Closes #38189 

- Fixed border dashed for diverse scenarios, as demonstrated in the
images below.
- This change has no impact on the rendering of solid borders, as it was
implemented inside an if block for dashed styles

Release Notes:
  - N/A

## Before Images
<details><summary>click to expand (small top border, medium right
border, large bottom border)</summary>
<img width="289" height="95" alt="Screenshot From 2025-09-15 13-28-14"
src="https://github.com/user-attachments/assets/5226cd0a-49c2-43b8-9df9-f64390e3759e"
/>
</details>
<details><summary> click to expand (Same size pairs of borders)
</summary>
<img width="289" height="95" alt="Screenshot From 2025-09-15 13-32-22"
src="https://github.com/user-attachments/assets/603e7b49-e8b1-45a4-ac35-1b3aedf52bca"
/>
<img width="289" height="95" alt="Screenshot From 2025-09-15 13-33-24"
src="https://github.com/user-attachments/assets/4243786c-4c9d-4419-91d6-4594b5ee4390"
/>
</details>

## After Images

<details><summary>click to expand (small top border, medium right
border, large bottom border)</summary>

<img width="289" height="95" alt="Screenshot From 2025-09-15 13-17-28"
src="https://github.com/user-attachments/assets/e2652b38-1c24-432e-b7fd-c6f4d4c71de6"
/>

</details>


<details><summary> click to expand (same size pairs of
borders)</summary>
<img width="289" height="95" alt="Screenshot From 2025-09-15 13-37-59"
src="https://github.com/user-attachments/assets/05228431-4a91-4531-adcd-d70acd2c3b44"
/>

<img width="289" height="95" alt="Screenshot From 2025-09-15 13-36-34"
src="https://github.com/user-attachments/assets/6da946b8-1ccd-4ed1-9b38-539eba4edf42"
/>
</details>
a22c29c5f9Francisco Gonzalez committed on 10/9/2025, 3:26:23 PM· committed by GitHubparentc543709
3 files changedLine totals unavailable