✅ tests: strengthen C++ binding smoke tests

[lalmei]

Summary:

• Expands the generated pybind11 smoke test to cover add(), version(), version, module docs, and TypeError behavior.
• Adds template-render assertions for the richer generated test.

Validation:

• uv run pytest -c=config/pytest.ini tests/test_template/test_cpp_template.py --no-cov
• make check
• make check-docs
• generated sample-cpp: make setup, make test, make format-check

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This is part 1 of 2 in a stack made with GitButler:

•  2  ✅ tests: strengthen C++ binding smoke tests #111 👈
•  1  ✨ template: expose C++ binding version #110