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/utils/sandbox-summary/src/config_summary.rs

@@ -28,7 +28,10 @@ pub fn create_config_summary_entries(config: &Config, model: &str) -> Vec<(&'sta
         ));
         entries.push((
             "reasoning summaries",
-            config.model_reasoning_summary.to_string(),
+            config
+                .model_reasoning_summary
+                .map(|summary| summary.to_string())
+                .unwrap_or_else(|| "none".to_string()),
         ));
     }