Use model catalog default for reasoning summary fallback (#12873)

## 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`

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/exec/src/lib.rs

@@ -560,7 +560,8 @@ pub async fn run_main(cli: Cli, arg0_paths: Arg0DispatchPaths) -> anyhow::Result
                     sandbox_policy: default_sandbox_policy.clone(),
                     model: default_model,
                     effort: default_effort,
-                    summary: default_summary,
+                    summary: default_summary
+                        .unwrap_or(codex_protocol::config_types::ReasoningSummary::Auto),
                     final_output_json_schema: output_schema,
                     collaboration_mode: None,
                     personality: None,