zeta: Refine visuals for the completion popover (#22142)

Most notably, trying out a different icon and adding the "Completion"
word to the side of the provider name.

<img width="800" alt="Screenshot 2024-12-17 at 13 04 55"
src="https://github.com/user-attachments/assets/6dcaa699-f358-4242-9812-e1668f426207"
/>

Release Notes:

- N/A

assets/icons/zed_predict.svg

@@ -1,4 +1,5 @@
 <svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
-<path d="M8 8.9V11C5.93097 11 5.06903 11 3 11V10.4L8 5.6V5H3V7.1" stroke="black" stroke-width="1.5"/>
-<path d="M11 5L13 8L11 11" stroke="black" stroke-width="1.5"/>
+<path d="M7 8.9V11C5.34478 11 4.65522 11 3 11V10.4L7 5.6V5H3V7.1" stroke="black" stroke-width="1.5"/>
+<path d="M12 5L14 8L12 11" stroke="black" stroke-width="1.5"/>
+<path d="M10 6.5L11 8L10 9.5" stroke="black" stroke-width="1.5"/>
 </svg>