cuda.core: initialize context in graph shutdown test

[rwgk]

Summary

• Establish a current CUDA context in the isolated graph-shutdown probe.
• Ensure the test reaches the interpreter-shutdown cleanup behavior it is
  intended to exercise.

Root Cause

test_pending_cleanup_is_safe_during_python_shutdown, added in #2371, runs
its probe in a fresh Python subprocess. The parent test's init_cuda fixture
does not provide a CUDA context to that subprocess. Constructing Device()
initializes CUDA, but does not make the device context current, so adding the
host callback node can fail with CUDA_ERROR_INVALID_VALUE before the shutdown
behavior is tested.

Explicitly call device.set_current() before constructing the graph.

Testing

• Focused graph-shutdown test passed.
• Full CUDA 13.4 Linux test run passed.
• Full CUDA 13.4 Windows test run passed.
• pre-commit run --all-files passed.

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