Builds on #17264.
- queues Realtime V2 `response.create` while an active response is open,
then flushes it after `response.done` or `response.cancelled`
- requests `response.create` after background agent final output and
steering acknowledgements
- adds app-server integration coverage for all `response.create` paths
Validation:
- `just fmt`
- `cargo check -p codex-app-server --tests`
- `git diff --check`
- CI green
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Co-authored-by: Codex <noreply@openai.com>