* Fix function attribute tests: __defaults__ del, __code__ free vars check, bound method dir()
- Support `del func.__defaults__` and `del func.__kwdefaults__` by changing
setter signatures to `PySetterValue<Option<...>>` so Delete is treated as
setting the value to None (matching CPython behaviour)
- Validate free-variable count when assigning to `func.__code__`: raise
ValueError when the new code object's freevars count doesn't match the
existing closure size
- Add `__dir__` to `PyBoundMethod` that delegates to the underlying
function's dir(), so attributes set on the function show up in
`dir(bound_method)`
- Remove `@unittest.expectedFailure` from the four tests that now pass:
test_blank_func_defaults, test_func_default_args, test___code__,
test_dir_includes_correct_attrs
* Remove expectedFailure markers for now-passing tests
