Preserve RNG state in dataset iteration

[pragnyanramtha]

Summary

• avoid resetting Python global random state while shuffling dataset epochs
• keep deterministic epoch ordering by using a local RNG seeded per epoch
• add focused tests for deterministic iteration and global RNG preservation

Validation

• uv run pytest tests/unit/test_iterate_dataset.py
• uv run ruff format --check src/art/utils/iterate_dataset.py tests/unit/test_iterate_dataset.py
• uv run ruff check src/art/utils/iterate_dataset.py tests/unit/test_iterate_dataset.py

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[Kovbo]

Good catch, thank you, @pragnyanramtha!