gh-145244: Fix use-after-free on borrowed dict key in json encoder (GH-145245)

In encoder_encode_key_value(), key is a borrowed reference from
PyDict_Next(). If the default callback mutates or clears the dict,
key becomes a dangling pointer. The error path then calls
_PyErr_FormatNote("%R", key) on freed memory.

Fix by holding strong references to key and value unconditionally
during encoding, not just in the free-threading build.

Co-authored-by: Peter Bierma <zintensitydev@gmail.com>