docs(hooks): document all usageMetadata token fields in LLMResponse reference

[AriaZhao-coder]

Summary

The hooks reference (docs/hooks/reference.md) documented LLMResponse.usageMetadata as containing a single field:

"usageMetadata": { "totalTokenCount": number }

However, the hook actually receives all three token fields. Both the LLMResponse interface and toHookLLMResponse() in packages/core/src/hooks/hookTranslator.ts populate promptTokenCount, candidatesTokenCount, and totalTokenCount.

This PR updates the reference so the documented shape matches what hooks really receive.

Change

-  "usageMetadata": { "totalTokenCount": number }
+  "usageMetadata": {
+    "promptTokenCount": number,
+    "candidatesTokenCount": number,
+    "totalTokenCount": number
+  }

Docs-only change; no code behavior is affected.

Fixes #28048

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request corrects the documentation for the LLMResponse interface to ensure it accurately describes the token metadata fields available to hooks. By updating the reference documentation, developers will have a clearer understanding of the data structure, preventing potential confusion regarding available token metrics.

Highlights

• Documentation Accuracy: Updated the LLMResponse documentation in docs/hooks/reference.md to correctly reflect the structure of the usageMetadata object.
• Metadata Field Expansion: Added missing promptTokenCount and candidatesTokenCount fields to the documentation to align with the actual data provided by the hook implementation.

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[github-actions]

📊 PR Size: size/XS

• Lines changed: 6
• Additions: +5
• Deletions: -1
• Files changed: 1

[gemini-code-assist]

Code Review

This pull request updates the documentation in docs/hooks/reference.md to expand the usageMetadata schema. Specifically, it adds promptTokenCount and candidatesTokenCount alongside totalTokenCount to provide a more detailed breakdown of token usage. There are no review comments, and I have no additional feedback to provide as the changes are straightforward and correct.

_{Note: Security Review has been skipped due to the limited scope of the PR.}