gh-146152: Fix memory leak in _json encoder error paths

Remove objects from markers dict on all error paths in
encoder_listencode_obj to prevent memory leaks when:
- default() raises an exception
- RecursionError occurs
- Nested encoding fails

Previously, objects were only removed on the success path,
leaving strong references in the markers dict.

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

@@ -1627,13 +1627,20 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
         }
         newobj = PyObject_CallOneArg(s->defaultfn, obj);
         if (newobj == NULL) {
-            Py_XDECREF(ident);
+            if (ident != NULL) {
+                PyDict_DelItem(s->markers, ident);
+                Py_XDECREF(ident);
+            }
             return -1;
         }
 
         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);
@@ -1642,7 +1649,10 @@ encoder_listencode_obj(PyEncoderObject *s, PyUnicodeWriter *writer,
         Py_DECREF(newobj);
         if (rv) {
             _PyErr_FormatNote("when serializing %T object", obj);
-            Py_XDECREF(ident);
+            if (ident != NULL) {
+                PyDict_DelItem(s->markers, ident);
+                Py_XDECREF(ident);
+            }
             return -1;
         }
         if (ident != NULL) {