test(python): add regression test for register_object __init__ wiring (#493)

## Summary

Add a regression test for the `register_object` `__init__` wiring that
prevents a NULL-handle segfault.

## Root Cause

`CObject` (Cython extension type) has a custom `tp_new`, so CPython's
`object.__init__` silently ignores extra positional arguments. Before
`_install_init` was wired into `register_object` (#491), calling
`TestIntPair(1, 2)` fell through to `object.__init__`, producing
`chandle = NULL`. Any subsequent field access (e.g. `pair.a`)
dereferences `chandle + field_offset` → **segfault**.

## Test Coverage

The new `test_register_object_wires_init` verifies:

1. Construction via the wired `__init__` yields a live (non-NULL) handle
with correct field values.
2. `__new__` alone leaves `chandle` at 0, confirming `__init__` is the
only path that creates the C++ object.

The test does not exercise the actual segfault (which would crash the
runner); the NULL-handle assertion is the safe proxy.

## Test Plan

- [x] `pytest -xvs
tests/python/test_object.py::test_register_object_wires_init` — passed