[3.13] gh-121153: Fix some errors with use of _PyLong_CompactValue() (GH-121154) (GH-121900)

* The result has type Py_ssize_t, not intptr_t.
* Type cast between unsigned and signed integer types should be explicit.
* Downcasting should be explicit.
* Fix integer overflow check in sum().
(cherry picked from commit 1801545)