tests: Improve automated tests for submessages.

Added an additional test case to `test_submessages.py` for testing the
message object containing `submessages` meta data.

Previous to this commit we were never validating the `submessage` schema
in the `message` objects.

Fixes #25896.

zerver/openapi/zulip.yaml

@@ -18080,7 +18080,7 @@ components:
                 type: integer
                 description: |
                   The ID of the user who sent the message.
-              submessage_id:
+              id:
                 type: integer
                 description: |
                   The ID of the submessage.

zerver/tests/test_submessage.py

@@ -200,3 +200,33 @@ class TestBasics(ZulipTestCase):
                     "Events should be sent only after the transaction commits."
                 )
                 do_add_submessage(hamlet.realm, hamlet.id, message_id, "whatever", "whatever")
+
+    def test_fetch_message_containing_submessages(self) -> None:
+        cordelia = self.example_user("cordelia")
+        stream_name = "Verona"
+        message_id = self.send_stream_message(
+            sender=cordelia,
+            stream_name=stream_name,
+        )
+        self.login_user(cordelia)
+
+        payload = dict(
+            message_id=message_id,
+            msg_type="whatever",
+            content='{"name": "alice", "salary": 20}',
+        )
+        self.assert_json_success(self.client_post("/json/submessage", payload))
+
+        result = self.client_get(f"/json/messages/{message_id}")
+        response_dict = self.assert_json_success(result)
+        self.assert_length(response_dict["message"]["submessages"], 1)
+
+        submessage = response_dict["message"]["submessages"][0]
+        expected_data = dict(
+            id=submessage["id"],
+            message_id=message_id,
+            content='{"name": "alice", "salary": 20}',
+            msg_type="whatever",
+            sender_id=cordelia.id,
+        )
+        self.assertEqual(submessage, expected_data)