gh-153852: Assert thread list non-empty only after stopping the world

Move the threads.head assert in handle_thread_shutdown_exception to after
_PyEval_StopTheWorld so it does not race tstate_delete_common (TSAN-0034).

Author: Vamsi-klu

Misc/NEWS.d/next/Core_and_Builtins/2026-07-26-12-00-00.gh-issue-153852.a1b2c3.rst

@@ -0,0 +1 @@
+Fix a free-threading race in finalization by asserting the thread list is non-empty only after stopping the world.

Python/pylifecycle.c

@@ -3827,8 +3827,8 @@ handle_thread_shutdown_exception(PyThreadState *tstate)
     assert(tstate != NULL);
     assert(_PyErr_Occurred(tstate));
     PyInterpreterState *interp = tstate->interp;
-    assert(interp->threads.head != NULL);
     _PyEval_StopTheWorld(interp);
+    assert(interp->threads.head != NULL);   /* safe: world is stopped */
 
     // We don't have to worry about locking this because the
     // world is stopped.