gh-134411: assert `PyLong_FromLong(x) != NULL` when `x` is known to be small (#134415)

Since `PyLong_From Long(PY_MONITORING_DEBUGGER_ID)` falls to `small_int` case and can't return `NULL`. Added `assert`s for extra confidence.
https://github.com/python/cpython/issues/134411#issuecomment-2897653868