Validate checkpoint GPU UUID inputs early

[aryanputta]

Summary

• reject non-UUID gpu_mapping values in cuda.core.checkpoint before they reach driver-facing restore args
• normalize malformed UUID string failures to the documented ValueError path
• add source-level checkpoint helper tests that run without importing the built cuda.core package

Testing

• python3 -m pytest cuda_core/tests/test_checkpoint_helpers.py --noconftest

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