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/rmcp_client.rs

@@ -398,6 +398,7 @@ async fn stdio_image_responses_are_sanitized_for_text_only_model() -> anyhow::Re
                 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,