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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---

Release Notes:

- N/A or Added/Fixed/Improved ...

crates/edit_prediction/src/zeta.rs

@@ -567,6 +567,7 @@ fn handle_api_response<T>(
 
 const ACTIVE_BUFFER_DIAGNOSTIC_ADDITIONAL_CONTEXT_TOKEN_COUNT: usize = 100;
 const MAX_ACTIVE_BUFFER_DIAGNOSTICS_TO_COLLECT: usize = 20;
+const MAX_ACTIVE_BUFFER_DIAGNOSTIC_MESSAGE_TOKENS_TO_COLLECT: usize = 512;
 const MAX_ACTIVE_BUFFER_DIAGNOSTIC_SNIPPET_TOKENS_TO_COLLECT: usize = 512;
 
 pub(crate) fn active_buffer_diagnostics(
@@ -601,7 +602,11 @@ pub(crate) fn active_buffer_diagnostics(
             };
             (
                 severity,
-                entry.diagnostic.message.clone(),
+                zeta_prompt::clamp_text_to_token_count(
+                    &entry.diagnostic.message,
+                    MAX_ACTIVE_BUFFER_DIAGNOSTIC_MESSAGE_TOKENS_TO_COLLECT,
+                )
+                .to_string(),
                 diagnostic_point_range,
                 snippet_point_range,
             )