Add unit tests for RL check_numbers

[hengtaoguo]

Description

Cover two test scenarios for utils_rl.check_numbers():

• b/490410887: Completions where numbers can/cannot be extracted by regex.
• b/490410887: Extracted numbers matching/not matching float-converted answer

Tests

CI

Checklist

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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