Add turn-scoped settings updates to the app server (#40656)

## What changed

- Add the experimental `turn/settings/update` method for changing `model`,
  `effort`, `summary`, and `serviceTier` on a specific live turn without
  changing future turns.
- Report whether the update was applied or the target turn was unavailable,
  and reject unsupported fields, disabled feature use, unsafe model changes,
  and direct updates to parent-owned subagents.
- Document the method's feature requirements, null handling, and limits.

## Testing

- Cover current-turn and future-turn isolation, rejected and unavailable
  targets, feature and capability gates, saved threads, and subagent ownership.

GitOrigin-RevId: 18440f6d7ee5fcd06ae23bd41b22c492b62bb2b5

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

@@ -54,6 +54,7 @@ const EXPERIMENTAL_CLIENT_METHOD_DEPENDENCY_TYPES: &[&str] = &[
     "ThreadBackgroundTerminal",
     "ThreadSearchOccurrence",
     "ThreadSearchTextRange",
+    "TurnSettingsUpdateStatus",
 ];
 const SPECIAL_DEFINITIONS: &[&str] = &[
     "ClientNotification",