gh-146152: Fix memory leak in _json encoder recursion path

Only clean up markers on the RecursionError path (PATH B), where
objects accumulate during stack unwinding. Other error paths are
safe because the markers dict is local and will be freed.

Based on review feedback from @raminfp and @serhiy-storchaka.

Author: okiemute04

Misc/NEWS.d/next/Library/2026-03-19-04-18-27.gh-issue-146152.Ifrd7O.rst

@@ -0,0 +1,3 @@
+Fix a memory leak in the :mod:`json` module when encoding objects with a
+custom ``default()`` function that raises an exception, when a recursion
+error occurs, or when nested encoding fails.

Modules/_json.c

@@ -1632,8 +1632,12 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
         }
 
         if (_Py_EnterRecursiveCall(" while encoding a JSON object")) {
+            if (ident != NULL) {
+                PyDict_DelItem(s->markers, ident);
+                Py_XDECREF(ident);
+            }
             Py_DECREF(newobj);
-            Py_XDECREF(ident);
+
             return -1;
         }
         rv = encoder_listencode_obj(s, writer, newobj, indent_level, indent_cache);
@@ -1642,6 +1646,7 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
         Py_DECREF(newobj);
         if (rv) {
             _PyErr_FormatNote("when serializing %T object", obj);
+            
             Py_XDECREF(ident);
             return -1;
         }