fix: avoid mutable default argument in ExactStringPerCategoryMatchMetric

[andrewwhitecdw]

Replaces the mutable default argument categories=[] with categories=None and instantiates a fresh list inside the constructor. This prevents shared state across metric instances.

• nemo/collections/common/metrics/classification_accuracy.py: changed default and added None guard
• Added regression tests in tests/collections/common/test_metrics.py

Tested:

uv run python -m pytest tests/collections/common/test_metrics.py::TestCommonMetrics::test_exact_string_per_category_match_metric_default_categories_is_none tests/collections/common/test_metrics.py::TestCommonMetrics::test_exact_string_per_category_match_metric_tracks_categories -v
# 2 passed

cc @nithinraok for common/core review

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[andrewwhitecdw]

Could a maintainer please add the Run CICD label to trigger CI? As an outside contributor I don't have permission to apply labels myself. Thanks!

[nithinraok]

These are all blind AI generated PRs. Why do we need a test case to test None as default?