Harden _typeconv.select_overload sequence handling

[fallintoplace]

Summary

• Harden numba_cuda/numba/cuda/cext/_typeconv.cpp select_overload() to use PySequence_Fast() on sigtup, ovsigstup, and each nested overload before item access.
• Replace unsafe PySequence_Fast_GET_ITEM usage on unchecked objects with explicit conversion, explicit error handling, and consistent cleanup.
• Add regression tests in numba_cuda/numba/cuda/tests/cudapy/test_typeconv.py for non-sequence inputs, invalid nested overload input, and mismatched overload arity.

Testing

• Not run in this workspace due environment/tooling constraints.

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