[feature] Implement reasoning_effort

[chaunceyjiang]

Purpose

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• 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 extends the reasoning_effort options to include 'xhigh' and refactors the logic for setting thinking_token_budget based on the chosen reasoning_effort. The review suggests improving the readability and maintainability of the new logic by using a dictionary lookup instead of an if/elif chain. Additionally, it highlights a potential regression where the include_reasoning=False setting for reasoning_effort='none' was removed and should be restored.