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/schema/json/ServerNotification.json

@@ -2585,6 +2585,12 @@
     },
     "ModelSafetyBufferingUpdatedNotification": {
       "properties": {
+        "fasterModel": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
         "model": {
           "type": "string"
         },
@@ -2594,6 +2600,9 @@
           },
           "type": "array"
         },
+        "showBufferingUi": {
+          "type": "boolean"
+        },
         "threadId": {
           "type": "string"
         },
@@ -2610,6 +2619,7 @@
       "required": [
         "model",
         "reasons",
+        "showBufferingUi",
         "threadId",
         "turnId",
         "useCases"