Adapt msgspec v4 pager payloads to msgspec v5 format

IPython/kernel/adapter.py

@@ -222,6 +222,14 b' class V4toV5(Adapter):'
         user_variables = content.pop('user_variables', {})
         if user_variables:
             user_expressions.update(user_variables)
+
+        # Pager payloads became a mime bundle
+        for payload in content.get('payload', []):
+            if payload.get('source', None) == 'page' and ('text' in payload):
+                if 'data' not in payload:
+                    payload['data'] = {}
+                payload['data']['text/plain'] = payload.pop('text')
+
         return msg
     
     def complete_request(self, msg):

IPython/kernel/tests/test_adapter.py

@@ -87,6 +87,22 b' class V4toV5TestCase(AdapterTest):'
         self.assertNotIn('user_variables', v5c)
         self.assertEqual(v5c['code'], v4c['code'])
     
+    def test_execute_reply(self):
+        msg = self.msg("execute_reply", {
+            'status': 'ok',
+            'execution_count': 7,
+            'user_variables': {'a': 1},
+            'user_expressions': {'a+a': 2},
+            'payload': [{'source':'page', 'text':'blah'}]
+        })
+        v4, v5 = self.adapt(msg)
+        v5c = v5['content']
+        self.assertNotIn('user_variables', v5c)
+        self.assertEqual(v5c['user_expressions'], {'a': 1, 'a+a': 2})
+        self.assertEqual(v5c['payload'], [{'source': 'page',
+                                           'data': {'text/plain': 'blah'}}
+                                         ])
+    
     def test_complete_request(self):
         msg = self.msg("complete_request", {
             'text' : 'a.is',