Fix CI: repair critical-section scoping in PyException_SetContext (gh-69968)

Py_END_CRITICAL_SECTION() emits a closing brace, so calling it inside the
nested if/while early-return path closed the wrong block and left
'base_self' undeclared after the loop. Restructure to a single exit:
record the cycle with a flag, then release the lock before Py_DECREF.

Also add the missing news entry.

Co-authored-by: Muse Code (Meta Muse Spark)

Author: ambv

Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-00-00-00.gh-issue-69968.3kX7mQ.rst

@@ -0,0 +1,3 @@
+Setting :attr:`~BaseException.__context__` to the exception itself now has no
+effect, and setting it to a chain containing the exception moves the
+exception to the start of the chain, preventing cycles.

Objects/exceptions.c

@@ -554,6 +554,8 @@ PyException_SetContext(PyObject *self, PyObject *context)
     assert(PyExceptionInstance_Check(self));
     assert(context == NULL || PyExceptionInstance_Check(context));
 
+    int self_in_chain = 0;
+
     Py_BEGIN_CRITICAL_SECTION(self);
     PyBaseExceptionObject *base_self = PyBaseExceptionObject_CAST(self);
 
@@ -568,18 +570,26 @@ PyException_SetContext(PyObject *self, PyObject *context)
                         base_self->context;
                 base_self->context = context;
             }
-            assert(Py_REFCNT(self) > 1);
-            Py_END_CRITICAL_SECTION();
-            Py_DECREF(self);
-            return;
+            self_in_chain = 1;
+            break;
         }
         assert(PyExceptionInstance_Check(exc));
         prev_exc = exc;
         exc = PyBaseExceptionObject_CAST(exc)->context;
     }
 
-    Py_XSETREF(base_self->context, context);
+    if (self_in_chain) {
+        /* Self was already in the chain; drop the lost link reference. */
+        assert(Py_REFCNT(self) > 1);
+    }
+    else {
+        Py_XSETREF(base_self->context, context);
+    }
     Py_END_CRITICAL_SECTION();
+
+    if (self_in_chain) {
+        Py_DECREF(self);
+    }
 }
 
 PyObject *