## Summary
- make `Config.model_reasoning_summary` optional so unset means use
model default
- resolve the optional config value to a concrete summary when building
`TurnContext`
- add protocol support for `default_reasoning_summary` in model metadata
## Validation
- `cargo test -p codex-core --lib client::tests -- --nocapture`
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Co-authored-by: Codex <noreply@openai.com>