Update Mistral AI provider icon (#49793)

This PR updates the Mistral AI provider icon to better align with
Mistral's branding. The current icon (from 2023) is not fully aligned
with Mistral's visual identity, and this change ensures consistency and
clarity for users.

### Changes Made
- Replaced the Mistral AI provider icon with an updated design to better
reflect Mistral's branding.

### Testing
- **Manual Testing:** Screenshots have been taken to verify the visual
changes. No visual regression testing was performed since this is a
minor icon update.
<img width="306" height="249" alt="Capture d’écran 2026-02-21 à 09 26
44"
src="https://github.com/user-attachments/assets/b6fa568b-66e5-4460-9bf8-88c0c6139c88"
/>
<img width="331" height="249" alt="Capture d’écran 2026-02-21 à 09 12
50"
src="https://github.com/user-attachments/assets/31ed4ffd-0b9a-47f3-92b8-e8eff6f0acb6"
/>
<img width="306" height="249" alt="Capture d’écran 2026-02-21 à 09 12
56"
src="https://github.com/user-attachments/assets/6339bb9e-5824-41e1-bdef-d809e5ac1cf3"
/>

- **Test Coverage:** No additional test coverage is required for this
change, as it is purely visual.

### Checklist
- [x] Added screenshots for manual testing.
- [x] Done a self-review taking into account security and performance
aspects (no impact).
- [x] Aligned any UI changes with the [UI
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist).

### Release Notes
- Updated Mistral AI provider icon to align with Mistral's branding.

---

### Additional Notes
- The previous icon (from 2024) was not fully aligned with Mistral's
visual identity. This update ensures consistency.
- Screenshots of the new icon are attached for review.

assets/icons/ai_mistral.svg

@@ -1,8 +1,12 @@
-<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path fill-rule="evenodd" clip-rule="evenodd" d="M10.4 4.4V6.8H9.2V4.4H10.4ZM12.8 2V4.4H11.6V2H12.8ZM2 2H3.2V14H2V2ZM6.8 9.2H8V11.6H6.8V9.2ZM11.6 9.2H12.8V14H11.6V9.2Z" fill="black"/>
-<path opacity="0.4" fill-rule="evenodd" clip-rule="evenodd" d="M12.8 2H15.2V4.4H12.8V2ZM3.2 2H5.6V4.4H3.2V2Z" fill="black"/>
-<path opacity="0.5" fill-rule="evenodd" clip-rule="evenodd" d="M10.4 6.8V4.4H15.2V6.8H10.4ZM3.2 6.8V4.4H8V6.8H3.2Z" fill="black"/>
-<path opacity="0.6" fill-rule="evenodd" clip-rule="evenodd" d="M3.2 9.2V6.8H15.2V9.2H3.2Z" fill="black"/>
-<path opacity="0.7" fill-rule="evenodd" clip-rule="evenodd" d="M8 9.2H10.4V11.6H8V9.2ZM12.8 9.2H15.2V11.6H12.8V9.2ZM3.2 9.2H5.6V11.6H3.2V9.2Z" fill="black"/>
-<path opacity="0.8" fill-rule="evenodd" clip-rule="evenodd" d="M12.8 11.6H15.2V14H12.8V11.6ZM3.2 11.6H5.6V14H3.2V11.6Z" fill="black"/>
+<svg width="16" height="16" xmlns="http://www.w3.org/2000/svg" version="1.1" viewBox="0 0 16 16">
+  <rect x="2.75" y="2.75" width="2.1" height="2.1"/>
+  <rect x="11.15" y="2.75" width="2.1" height="2.1"/>
+  <rect x="6.95" y="9.05" width="2.1" height="2.1"/>
+  <rect x="2.75" y="9.05" width="2.1" height="2.1"/>
+  <rect x="11.15" y="9.05" width="2.1" height="2.1"/>
+  <rect x="2.75" y="4.85" width="4.2" height="2.1"/>
+  <rect x=".65" y="11.15" width="6.3" height="2.1"/>
+  <rect x="9.05" y="11.15" width="6.3" height="2.1"/>
+  <rect x="2.75" y="6.95" width="10.49" height="2.1"/>
+  <rect x="9.05" y="4.85" width="4.2" height="2.1"/>
 </svg>