Add a new goodput elastic utils function record_slice_state()

[YixuanWang-99]

Update MaxText's utils/elastic_utils.py to add a new record_slice_state helper that queries live slice counts and logs them to the GoodputRecorder. This helper maps the core metrics directly from Pathways and JAX states.

Tests

Passed synthetic tests in tests/unit/elastic_utils_test.py.

Test with a small synthetic workflow and the logs are good:

Checklist

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• 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

❌ Patch coverage is 52.63158% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/common/goodput.py	33.33%	13 Missing and 3 partials ⚠️
src/maxtext/utils/elastic_utils.py	85.71%	1 Missing and 1 partial ⚠️

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