[Frontend] Fixes anthropic streaming message_start usage nesting

[bbartels]

Purpose

Fixes anthropic streaming message_start usage nesting. Follow up to: #29971

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
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• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[gemini-code-assist]

Code Review

This pull request correctly adjusts the nesting of the usage field within the message_start event for Anthropic API streaming responses. The change moves the usage object to be a child of the message object, which aligns the server's output with the official Anthropic API specification. The corresponding test case has also been updated to reflect this structural change. The implementation is correct and effectively resolves the issue.

[mergify]

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