Use pathlib in cuda.core build hooks, tests and examples (part 5 of #2410)

[LeSingh1]

Part 5 of #2410.

Replaces os.path with pathlib.Path in cuda_core/build_hooks.py, tests/helpers/__init__.py, the example test driver, and the two examples that assemble CUDA include paths.

ProgramOptions.include_path is typed str | list[str] | tuple[str], so the values handed to it stay str; only the path construction moves to pathlib. Same for the Extension arguments.

The one non-mechanical change is extension discovery, which previously sliced glob result strings against an os.path.sep-built prefix. It now relative-paths against Path("cuda", "core") and yields POSIX-style module names on every platform — which is what the old mod.replace(os.path.sep, "/") normalization already did. I checked that by re-implementing the old and new logic side by side and diffing them against the real source tree on Linux: identical module names, Extension names, source tuples and arch-specific sources across all 45 modules.

glob.glob is left as-is throughout; only the pattern construction moved. os.path.isdir → Path.is_dir() was checked per call rather than bulk-replaced — each one guards a "does this include/lib dir exist" decision where both spellings return False for a broken symlink or a permission error.

Verified with cuda_core/tests/test_build_hooks.py on Linux CI: 14 passed, unchanged from the baseline. The test modules and examples need a GPU or a built cuda.bindings, so those changes are by inspection plus ruff and py_compile; they are strictly mechanical.

NOTE: developed with the assistance of an AI coding agent. I reviewed and verified the change before submitting.

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