Validate GPU split bucket indices before slicing

[mikemikimike]

Fixes #284

Issue: #284

Summary

• Validate GPU split index and bucket tensors before using bucket values for row selection.
• Keep model layer state unchanged until every layer passes validation.
• Handle invalid slice allocation results without dereferencing null tensors.
• Add regression coverage for valid, empty, negative, out-of-range, oversized, and zero-row buckets.

Testing

• cmake -S . -B build-prepr -DCMAKE_BUILD_TYPE=Release -DLLAMA_BUILD_TESTS=ON
• cmake --build build-prepr --parallel 2
• ctest --test-dir build-prepr --output-on-failure (19/19 passed)