test(cpp): assert a union member function becomes a method edge #4070

Good first issue. Union support landed in #4052, but no test covers a union with a member function.

Where: tests/test_cpp_union_specifier_is_extracted (tests/test_cpp_*.py)
What: add a case with a named union that declares a method, e.g. union U { int i; int get() const; };, and assert get is emitted as a method edge of U (not a field).
Acceptance: new test passes with uv run pytest -k union.