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.

Checklist

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[codecov]

Codecov Report

❌ Patch coverage is 9.09091% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/maxtext/utils/elastic_utils.py	9.09%	10 Missing ⚠️

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