gh-159739: Clear free-threaded GC worklists after early errors (#159979)

Backport to 3.15 and 3.14.

Author: jelle-openai

Misc/NEWS.d/next/Core_and_Builtins/2026-10-20-00-00-00.gh-issue-159739.da0f04.rst

@@ -0,0 +1,2 @@
+Fix reference leaks when free-threaded garbage collection aborts early because
+of an allocation error.

Python/gc_free_threading.c

@@ -2123,6 +2123,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
         int err = gc_mark_alive_from_roots(interp, state);
         if (err < 0) {
             _PyEval_StartTheWorld(interp);
+            cleanup_worklist(&state->objs_to_decref);
             PyErr_NoMemory();
             return;
         }
@@ -2133,6 +2134,9 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state,
     int err = deduce_unreachable_heap(interp, state);
     if (err < 0) {
         _PyEval_StartTheWorld(interp);
+        cleanup_worklist(&state->unreachable);
+        cleanup_worklist(&state->legacy_finalizers);
+        cleanup_worklist(&state->objs_to_decref);
         PyErr_NoMemory();
         return;
     }