Harden _typeconv.select_overload sequence handling

[fallintoplace]

Summary

• Harden numba_cuda/numba/cuda/cext/_typeconv.cpp select_overload() to validate and materialize sequence arguments with PySequence_Fast() before indexed access.
• Add error checks for invalid outer/nested sequence inputs and mismatched overload arity.
• Add regression tests in numba_cuda/numba/cuda/tests/cudapy/test_typeconv.py covering raw C-extension invalid input paths.

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