cuda.core: Fix graphics tests

[juenglin]

Description

The GL context helpers in cuda_core/tests/test_graphics.py and cuda_bindings/tests/test_graphics_apis.py wrapped yield in except Exception and called pytest.skip(), so CUDA errors, assertion failures, and TypeErrors raised in the test body were recorded as "could not create GL context" skips rather than failures. The cuda_core graphics tests also never requested init_cuda, so whether from_gl_buffer / from_gl_image succeeded depended on leftover thread state from the previous test (pytest-randomly seed).

This PR:

• Splits each helper into a plain setup function (_configure_pyglet_headless, _open_gl_window, _setup_gl_buffer, _setup_gl_texture) and a thin context manager that wraps only the setup call in try/except so test-body exceptions propagate as failures.
• Adds init_cuda to every cuda_core graphics test that touches CUDA, and uses init_cuda.create_stream() instead of a local _create_stream() helper.
• Converts the cuda_core test classes to plain test functions (they carried no shared state or fixtures).
• Fixes two StridedMemoryView.from_buffer call sites that passed dtype=np.float32 (a scalar type class) instead of np.dtype(np.float32).
• Marks test_close_while_mapped_passes_stream_override as xfail(strict=True) because Buffer is an immutable Cython type and neither patch.object nor __class__ assignment can intercept Buffer.close.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[juenglin]

/ok to test baf3e23

[github-actions]

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[juenglin]

I couldn't get this test to work. Should I remove it?

============================================================= short test summary info =============================================================
FAILED cuda_core/tests/test_graphics.py::test_close_while_mapped_passes_stream_override - TypeError: cannot set 'close' attribute of immutable type 'cuda.core._memory._buffer.Buffer'
========================================================== 1 failed, 29 passed in 0.63s ===========================================================

[juenglin]

/ok to test 8fbb9cf

[juenglin]

pre-commit.ci run

[juenglin]

/ok to test 5dc9814