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/resume_warning.rs

@@ -4,6 +4,7 @@ use codex_core::CodexAuth;
 use codex_core::NewThread;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ModeKind;
+use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::ResumedHistory;
@@ -34,7 +35,9 @@ fn resume_history(
         personality: None,
         collaboration_mode: None,
         effort: config.model_reasoning_effort,
-        summary: config.model_reasoning_summary,
+        summary: config
+            .model_reasoning_summary
+            .unwrap_or(ReasoningSummary::Auto),
         user_instructions: None,
         developer_instructions: None,
         final_output_json_schema: None,