test: centralize Pyglet GL window setup

[rwgk]

Description

Follow-up to #2813.

Centralize Pyglet GL window setup in cuda_python_test_helpers.graphics and use it from both the cuda_core and cuda_bindings graphics tests. This shares partial-window cleanup and previous-context restoration across both suites while keeping Pyglet imports lazy so callers can configure headless mode first.

Add one focused regression test covering failed window construction, cleanup of the partial window, and restoration of the previous context.

Testing

• Changed-file pre-commit checks pass.
• The focused graphics suites collect 32 tests successfully.
• The lifecycle regression scenario passes independently.
• Full GPU test execution is deferred to CI because the local host reports CUDA_ERROR_NO_DEVICE.

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes. No documentation changes are needed.

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[rwgk]

/ok to test

[rwgk]

Thanks @mdboom!

@rluo8 for visibility

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