[Structured Output] Make the output of structured output example more complete

[shen-shanshan]

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
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Currently, the default length of the output in structured output example is short and the result can be incomplete.

Guided decoding by Choice: Positive
Guided decoding by Regex: alan_turing@enigma.com
# Incomplete outputs:
Guided decoding by JSON: {"brand": "Porsche", "model": "911", "
Guided decoding by Grammar: SELECT col_1  from table_1  where col

Add max_tokens to sampling_params to make the output more complete:

# ...
Guided decoding by JSON: {"brand": "Porsche", "model": "911", "car_type": "sedan"}
Guided decoding by Grammar: SELECT col_1  from table_1  where col_2 = 1

Test Plan

Test Result

(Optional) Documentation Update

[gemini-code-assist]

Code Review

This pull request addresses an issue where the structured output example could produce incomplete results. By introducing a MAX_TOKENS constant and applying it to the sampling parameters for regex, JSON, and grammar-guided decoding, the change ensures that the generated outputs are complete. The implementation is clean and effectively solves the problem described. The code looks good, and I have no further comments.

[github-actions]

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