editor: Mute completion detail text (#64684)

# Objective

Fixes #49883.

## Solution

Render completion detail suffixes with the muted text color while
preserving syntax highlights.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

| Before | After |
| --- | --- |
| ![Completion details before the
change](https://github.com/user-attachments/assets/d778761a-bff8-4186-ae4c-a0daca6fcbf3)
| ![Completion details after the
change](https://github.com/user-attachments/assets/4a9c05de-8606-4d12-bbb3-7d15e0afc7fe)
|

---

Release Notes:

- Fixed completion details lacking visual distinction from completion
labels.
5402570caaGautam Manchandani committed on 9/24/2026, 2:56:42 PM· committed by GitHubparent6fae7f3
1 file changedLine totals unavailable