Fix zero reduce bucket size validation

[fwerkor]

Summary

Reject zero_optimization.reduce_bucket_size=0 during configuration validation instead of allowing ZeRO to fail later with a division by zero during backward.

This changes the field constraint from non-negative to strictly positive and adds a boundary regression test covering 1 as valid and 0 as invalid.

Fixes #8260

Testing

• python -m pytest tests/unit/runtime/zero/test_zero_config.py -q (7 passed)
• pre-commit run --files deepspeed/runtime/zero/config.py tests/unit/runtime/zero/test_zero_config.py

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[fwerkor]

Hi @tohtana, gentle ping — would appreciate a review when you have a chance. Thanks!