Fix cp_gather_cache block table bounds

[johnnyychiu]

Purpose

Fixes #45462.

Prevents seq_starts from driving cp_gather_cache past the end of a block_table row. The kernel now checks the computed block-table index before reading the row and writes zeros for out-of-range cache spans instead of reading an invalid block id. The related gather_and_maybe_dequant_cache path uses the same bounds check, since it has the same seq_starts indexing pattern.

This also widens the offset arithmetic around seq_starts to avoid overflowing before the bounds check.

Test Plan

• Add a regression test for the existing gather_and_maybe_dequant_cache OOB case.
• Add a regression test for the cp_gather_cache OOB case from [Bug]: Out of bounds in cp_gather_cache #45462.
• Run local whitespace and Python syntax checks.

Test Result

• git diff --check passed.
• python.exe -m py_compile tests\kernels\test_cache_kernels.py passed.
• Not run: python.exe -m pytest tests\kernels\test_cache_kernels.py because the available local Python runtime does not have pytest installed.

[github-actions]

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