Port parsing fallback from evaluation to rewards

[A9isha]

Description

Look for last <answer> </answer> even in reward functions

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Tests

CI and I ran on v7x-128

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/trainers/post_train/rl/utils_rl.py	50.00%	8 Missing ⚠️

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[hengtaoguo]

LGTM! Could you also check why the tests are failing? If we update the reward logics in the PR, the expected reward values need to be updated too.

FAILED tests/post_training/unit/rl_utils_test.py::TestCheckNumbers::test_extraction_fails_answer_tags_only - AssertionError: 1.5 != 0