perf: clear generated primitive arrays with slice deletion

[Lidang-Jiang]

Summary

Replace the generated Python array reset loop with PySequence_DelSlice() when converting primitive sequences to Python, and add regression coverage for bounded and unbounded primitive sequence conversion.

Closes #255

Before

$ git show upstream/rolling:rosidl_generator_py/resource/_msg_support.c.em | sed -n "675,705p"
@(bi)    Py_DECREF(itemsize_attr);
@(bi)    if (itemsize != sizeof(@primitive_msg_type_to_c(member.type.value_type))) {
@(bi)      PyErr_SetString(PyExc_RuntimeError, "itemsize doesn't match expectation");
@(bi)      Py_DECREF(field);
@(bi)      return NULL;
@(bi)    }
@(bi)    // clear the array, poor approach to remove potential default values
@(bi)    Py_ssize_t length = PyObject_Length(field);
@(bi)    if (-1 == length) {
@(bi)      Py_DECREF(field);
@(bi)      return NULL;
@(bi)    }
@(bi)    if (length > 0) {
@(bi)      PyObject * pop = PyObject_GetAttrString(field, "pop");
@(bi)      assert(pop != NULL);
@(bi)      for (Py_ssize_t i = 0; i < length; ++i) {
@(bi)        PyObject * ret = PyObject_CallFunctionObjArgs(pop, NULL);
@(bi)        if (!ret) {
@(bi)          Py_DECREF(pop);
@(bi)          Py_DECREF(field);
@(bi)          return NULL;
@(bi)        }
@(bi)        Py_DECREF(ret);
@(bi)      }
@(bi)      Py_DECREF(pop);
@(bi)    }
@(bi)    if (ros_message->@(member.name).size > 0) {
@(bi)      // populating the array.array using the frombytes method
@(bi)      PyObject * frombytes = PyObject_GetAttrString(field, "frombytes");
@(bi)      assert(frombytes != NULL);
@(bi)      @primitive_msg_type_to_c(member.type.value_type) * src = &(ros_message->@(member.name).data[0]);

$ git show upstream/rolling:rosidl_generator_py/test/test_interfaces.py | rg -n "convert_to_py|PyCapsule|primitive_sequences_convert" || true

After

$ git diff --check

$ python3 -m py_compile rosidl_generator_py/test/test_interfaces.py
$WORKSPACE/python310_torch25_cuda/lib/python3.10/site-packages/xpytorch_import_hook.py:6: UserWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html. The pkg_resources package is slated for removal as early as 2025-11-30. Refrain from using this package or pin to Setuptools<81.
  import pkg_resources

$ ruff check rosidl_generator_py/test/test_interfaces.py
All checks passed!

$ rg -n "PySequence_DelSlice|primitive_sequences_convert_to_py|PyCapsule_GetPointer" rosidl_generator_py/resource/_msg_support.c.em rosidl_generator_py/test/test_interfaces.py
rosidl_generator_py/test/test_interfaces.py:44:_PY_CAPSULE_GET_POINTER = ctypes.pythonapi.PyCapsule_GetPointer
rosidl_generator_py/test/test_interfaces.py:823:def test_primitive_sequences_convert_to_py(
rosidl_generator_py/resource/_msg_support.c.em:687:@(bi)    if (PySequence_DelSlice(field, 0, length) == -1) {

$ python -m pytest -q rosidl_generator_py/test/test_interfaces.py::test_primitive_sequences_convert_to_py
ERROR: found no collectors for $WORKSPACE/ros2-contrib/worktrees/rosidl_python-255/rosidl_generator_py/test/test_interfaces.py::test_primitive_sequences_convert_to_py


==================================== ERRORS ====================================
___________________ ERROR collecting test/test_interfaces.py ___________________
ImportError while importing test module '$WORKSPACE/ros2-contrib/worktrees/rosidl_python-255/rosidl_generator_py/test/test_interfaces.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
../../../python310_torch25_cuda/lib/python3.10/importlib/__init__.py:126: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
rosidl_generator_py/test/test_interfaces.py:24: in <module>
    from rosidl_generator_py.msg import Arrays
E   ModuleNotFoundError: No module named 'rosidl_generator_py.msg'
=========================== short test summary info ============================
ERROR rosidl_generator_py/test/test_interfaces.py
1 error in 0.10s

Test plan

• git diff --check
• python3 -m py_compile rosidl_generator_py/test/test_interfaces.py
• ruff check rosidl_generator_py/test/test_interfaces.py
• Target pytest cannot complete in this local environment because rosidl_generator_py.msg is generated by the ROS build/test setup and is not present in this source checkout