Fix memory leak: clear managed dict in pybind11_object_dealloc on Python 3.13+

[yamedvedya]

Closes #5998

Problem

On Python 3.14, objects stored in the __dict__ of py::dynamic_attr() instances are permanently leaked — their refcounts are abandoned when the pybind11 object is freed, so destructors (including capsule destructors) never run.

Root cause: pybind11_object_dealloc() calls type->tp_free(self) without first calling PyObject_ClearManagedDict(self). On Python ≤ 3.13, PyObject_GC_Del (what tp_free resolves to for GC-tracked objects) cleared the managed dict as a side effect. On Python 3.14 this implicit clearing was removed, requiring an explicit call in tp_dealloc.

The existing pybind11_clear() (which correctly calls PyObject_ClearManagedDict) is only invoked by the cyclic GC via tp_clear — it is never called during normal reference-count-driven deallocation.

Fix

// pybind11/detail/class.h — pybind11_object_dealloc()

  #if PY_VERSION_HEX >= 0x030D0000
      if (PyType_HasFeature(type, Py_TPFLAGS_MANAGED_DICT)) {
          PyObject_ClearManagedDict(self);
      }
  #endif

PyObject_ClearManagedDict is idempotent, so calling it before tp_free is safe on all Python 3.13+ versions.

Test

Added test_dynamic_attr_dealloc_frees_dict_contents to test_methods_and_attributes:

• Creates a DynamicClass (py::dynamic_attr()) instance
• Stores a py::capsule in its __dict__; the capsule destructor sets a global flag
• Asserts the flag is set after del instance + gc_collect()

Impact

Any py::dynamic_attr() class that stores Python objects in instance __dict__ from C++ (via obj.attr("x") = value) is affected. A prominent real-world case is https://gitlab.com/tango-controls/pytango, where
Tango::DeviceAttribute uses py::dynamic_attr() to store zero-copy capsule-backed numpy arrays — these arrays leaked completely under Python 3.14 (https://gitlab.com/tango-controls/pytango/-/issues/744).