refactor(tests): replace exec() with importlib in example runner

[OfficialSerge]

Description

This PR is in response to the following TODO item in cuda_core/tests/example_tests/utils.py.

# TODO: Refactor the examples to give them a common callable `main()` to avoid needing to use exec here?
exec(script, env if env else {})

Key Changes

• refactor run_example in utils.py using importlib, adding support for both main() and exec() scripts, tracebacks should now provide real line number feedback, thus paving the way for us to progressively enhance the remaining non-module tests and examples.
• updated test_basic_example.py to use pathlib.Path instead of os.path.join().
• removed redundant manual Device(0) and set_current() calls in test class, instead relying on exiting deinit_cuda() fixture for consistent teardowns (see conftest.py)

Next Steps

• go through the remaining examples still without main() and refactor them into Python modules, some examples are currently refactored while others are still plain executables so it's a mix.

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[mdboom]

I don't think we necessarily need to duplicate information from the Python stdlib docs here, but don't feel strongly either way.

[OfficialSerge]

I see, my ruff linter was flagging it as a potential point of runtime failure, the if-none check fixed these issues though I figured I'd leave an explanation to help people understand the need for the check, in case there are any future revisions.