Delete unused ResponseItem::Message.end_turn (#19605)

This field is unused. Delete it.

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -9413,12 +9413,6 @@
               },
               "type": "array"
             },
-            "end_turn": {
-              "type": [
-                "boolean",
-                "null"
-              ]
-            },
             "id": {
               "type": [
                 "string",
@@ -15643,4 +15637,4 @@
   },
   "title": "CodexAppServerProtocolV2",
   "type": "object"
-}
+}