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/core/tests/suite/rmcp_client.rs

@@ -47,10 +47,7 @@ async fn stdio_server_round_trip() -> anyhow::Result<()> {
         &server,
         any(),
         responses::sse(vec![
-            serde_json::json!({
-                "type": "response.created",
-                "response": {"id": "resp-1"}
-            }),
+            responses::ev_response_created("resp-1"),
             responses::ev_function_call(call_id, &tool_name, "{\"message\":\"ping\"}"),
             responses::ev_completed("resp-1"),
         ]),
@@ -184,10 +181,7 @@ async fn streamable_http_tool_call_round_trip() -> anyhow::Result<()> {
         &server,
         any(),
         responses::sse(vec![
-            serde_json::json!({
-                "type": "response.created",
-                "response": {"id": "resp-1"}
-            }),
+            responses::ev_response_created("resp-1"),
             responses::ev_function_call(call_id, &tool_name, "{\"message\":\"ping\"}"),
             responses::ev_completed("resp-1"),
         ]),
@@ -352,10 +346,7 @@ async fn streamable_http_with_oauth_round_trip() -> anyhow::Result<()> {
         &server,
         any(),
         responses::sse(vec![
-            serde_json::json!({
-                "type": "response.created",
-                "response": {"id": "resp-1"}
-            }),
+            responses::ev_response_created("resp-1"),
             responses::ev_function_call(call_id, &tool_name, "{\"message\":\"ping\"}"),
             responses::ev_completed("resp-1"),
         ]),