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/core/tests/suite/model_switching.rs

@@ -234,6 +234,7 @@ async fn model_change_from_image_to_text_strips_prior_image_content() -> Result<
         base_instructions: "base instructions".to_string(),
         model_messages: None,
         supports_reasoning_summaries: false,
+        default_reasoning_summary: ReasoningSummary::Auto,
         support_verbosity: false,
         default_verbosity: None,
         apply_patch_tool_type: None,
@@ -391,6 +392,7 @@ async fn model_switch_to_smaller_model_updates_token_context_window() -> Result<
         base_instructions: "base instructions".to_string(),
         model_messages: None,
         supports_reasoning_summaries: false,
+        default_reasoning_summary: ReasoningSummary::Auto,
         support_verbosity: false,
         default_verbosity: None,
         apply_patch_tool_type: None,