Add helper for response created SSE events in tests (#4758)

## Summary
- add a reusable `ev_response_created` helper that builds
`response.created` SSE events for integration tests
- update the exec and core integration suites to use the new helper
instead of repeating manual JSON literals
- keep the streaming fixtures consistent by relying on the shared helper
in every touched test

## Testing
- `just fmt`


------
https://chatgpt.com/codex/tasks/task_i_68e1fe885bb883208aafffb94218da61

codex-rs/exec/tests/suite/output_schema.rs

@@ -24,10 +24,7 @@ async fn exec_includes_output_schema_in_request() -> anyhow::Result<()> {
 
     let server = responses::start_mock_server().await;
     let body = responses::sse(vec![
-        serde_json::json!({
-            "type": "response.created",
-            "response": {"id": "resp1"}
-        }),
+        responses::ev_response_created("resp1"),
         responses::ev_assistant_message("m1", "fixture hello"),
         responses::ev_completed("resp1"),
     ]);