Stabilize compact rollback follow-up test

[dylan-hurd-oai]

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