gh-144295: Fix data race in dict method lookup and global load (gh-144312)

In `_PyDict_GetMethodStackRef`, only use the fast-path unicode lookup
when the dict is owned by the current thread or already marked as shared.
This prevents a race between the lookup and concurrent dict resizes,
which may free the PyDictKeysObject (i.e., it ensures that the resize
uses QSBR).

Address a similar issue in `_Py_dict_lookup_threadsafe_stackref` by
calling `ensure_shared_on_read()`.