Add unit tests for check_answer

[Shuwen-Fang]

Description

Add unit tests for check_answer to cover the following cases:

• Correct, whitespace-stripped, ratio-threshold, and incorrect answers
• DAPO dataset_name triggers normalize_final_answer
• Completions where match_format_regex finds no match

Tests

pytest -rs tests/unit/rl_utils_test.py passes

Checklist

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• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[hengtaoguo]

You may need to reformat this file because of the pyink error.

Try pyink --pyink-indentation=2 --line-length=122 ., only keep the changes in rl_utils_test.py and revert the others for now.