Stabilize compact rollback follow-up test (#22303)

## Summary
- add the missing response.created event to the mocked empty follow-up
response in the compact rollback test
- keep the fix scoped to the flaky mocked stream shape, without
increasing timeouts

## Recent flakes on main
- `snapshot_rollback_followup_turn_trims_context_updates` failed in
`rust-ci-full` on `main` in the Ubuntu remote test job on 2026-05-14:
https://github.com/openai/codex/actions/runs/25891434395/job/76095284830
- The same `compact_resume_fork` suite also failed recently on `main`
with `snapshot_rollback_past_compaction_replays_append_only_history`,
which has the same mocked Responses stream shape sensitivity this PR is
tightening:
https://github.com/openai/codex/actions/runs/25892437363/job/76098329098

## Verification
- env -u CODEX_SANDBOX_NETWORK_DISABLED cargo test -p codex-core --test
all snapshot_rollback_followup_turn_trims_context_updates -- --nocapture
- repeated the same focused test 3 consecutive times locally
- UV_CACHE_DIR=/private/tmp/uv-cache-codex-fmt just fmt

codex-rs/core/tests/suite/compact_resume_fork.rs

@@ -30,6 +30,7 @@ use core_test_support::responses::ResponseMock;
 use core_test_support::responses::ResponsesRequest;
 use core_test_support::responses::ev_assistant_message;
 use core_test_support::responses::ev_completed;
+use core_test_support::responses::ev_response_created;
 use core_test_support::responses::mount_sse_once_match;
 use core_test_support::responses::mount_sse_sequence;
 use core_test_support::responses::sse;
@@ -535,7 +536,7 @@ async fn snapshot_rollback_followup_turn_trims_context_updates() -> Result<()> {
                 ev_assistant_message("m2", "turn 2 assistant"),
                 ev_completed("r2"),
             ]),
-            sse(vec![ev_completed("r3")]),
+            sse(vec![ev_response_created("r3"), ev_completed("r3")]),
         ],
     )
     .await;