ep: Limit diagnostic message token counts (#59644)

# Objective

Prevent unbounded ep request sizes

## Solution

Use same token limiting logic used for snippets, on diagnostic messages

## Testing

- Did you test these changes? If so, how?
- Are there any parts that need more testing?
- How can other people (reviewers) test your changes? Is there anything
specific they need to know?
- If relevant, what platforms did you test these changes on, and are
there any important ones you can't test?

## 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
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

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or does not add a new user-facing feature, you can delete this section.

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- A before/after comparison is very useful for changes to existing
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  <summary>Click to view showcase</summary>

My super cool demos here

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---

Release Notes:

- N/A or Added/Fixed/Improved ...
45e84381e2Ben Kunkle committed on 6/20/2026, 8:57:43 PM· committed by GitHubparentf99df1a
1 file changedLine totals unavailable