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/codex-api/src/common.rs

@@ -118,6 +118,24 @@ pub enum ResponseEvent {
 pub struct SafetyBuffering {
     pub use_cases: Vec<String>,
     pub reasons: Vec<String>,
+    #[serde(skip)]
+    pub show_buffering_ui: bool,
+    #[serde(skip)]
+    pub faster_model: Option<String>,
+}
+
+impl SafetyBuffering {
+    pub(crate) fn with_treatment(mut self, treatment: &SafetyBufferingTreatment) -> Self {
+        self.show_buffering_ui = treatment.show_buffering_ui;
+        self.faster_model.clone_from(&treatment.faster_model);
+        self
+    }
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub(crate) struct SafetyBufferingTreatment {
+    pub show_buffering_ui: bool,
+    pub faster_model: Option<String>,
 }
 
 #[derive(Debug, Serialize, Clone, PartialEq)]