Propagate safety buffering treatment metadata (#29473)

## Summary

- read the request-scoped safety-buffering treatment from HTTP response
headers and per-turn WebSocket metadata through one shared header parser
- combine that treatment with Responses API safety-buffering signals
- propagate `showBufferingUi` and nullable `fasterModel` through the
existing `model/safetyBuffering/updated` app-server notification
- update the app-server documentation and generated JSON and TypeScript
schemas

The public implementation contains no model mapping or real model
identifier. Tests and protocol examples use generic `current-model` and
`faster-model` placeholders only.

## Dependencies

- server-side treatment evaluation:
https://github.com/openai/openai/pull/1060247
- initial Responses API safety-buffering propagation:
https://github.com/openai/codex/pull/29371
- Codex App UI: https://github.com/openai/openai/pull/1057789

## Validation

- Codex API tests: 129 passed
- focused Codex core safety-buffering integration test passed
- app-server protocol tests passed after regenerating schema fixtures
- Clippy fix and repository formatting completed successfully

The broader app-server run compiled all changed crates and completed
with 1,269 passing tests. Its remaining failures were unrelated
environment limitations: macOS sandbox application was denied, one
expected test binary was unavailable, and several existing subprocess
tests timed out as a result.

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -3392,9 +3392,11 @@ mod tests {
             v2::ModelSafetyBufferingUpdatedNotification {
                 thread_id: "thr_123".to_string(),
                 turn_id: "turn_123".to_string(),
-                model: "gpt-5.4".to_string(),
+                model: "current-model".to_string(),
                 use_cases: vec!["cyber".to_string()],
                 reasons: vec!["user_risk".to_string()],
+                show_buffering_ui: true,
+                faster_model: Some("faster-model".to_string()),
             },
         );
         assert_eq!(
@@ -3403,9 +3405,11 @@ mod tests {
                 "params": {
                     "threadId": "thr_123",
                     "turnId": "turn_123",
-                    "model": "gpt-5.4",
+                    "model": "current-model",
                     "useCases": ["cyber"],
-                    "reasons": ["user_risk"]
+                    "reasons": ["user_risk"],
+                    "showBufferingUi": true,
+                    "fasterModel": "faster-model"
                 }
             }),
             serde_json::to_value(&notification)?,

codex-rs/app-server-protocol/src/protocol/v2/model.rs

@@ -172,4 +172,6 @@ pub struct ModelSafetyBufferingUpdatedNotification {
     pub model: String,
     pub use_cases: Vec<String>,
     pub reasons: Vec<String>,
+    pub show_buffering_ui: bool,
+    pub faster_model: Option<String>,
 }