Add thread/settings/update app-server API (#23502)

## Why

App-server clients need a way to update a thread's next-turn settings
without starting a turn, adding transcript content, or waiting for turn
lifecycle events. This gives settings UI a direct path for durable
thread settings while clients observe the eventual effective state
through a notification.

This is a simplified rework of PR
https://github.com/openai/codex/pull/22509. In particular, it changes
the `thread/settings/update` api to return immediately rather than
waiting and returning the effective (updated) thread settings. This
makes the new api consistent with `turn/start` and greatly reduces the
complexity of the implementation relative to the earlier attempt.

## What Changed

- Adds experimental `thread/settings/update` with partial-update request
fields and an empty acknowledgment response.
- Adds experimental `thread/settings/updated`, carrying full effective
`ThreadSettings` and scoped by `threadId` to subscribed clients for the
affected thread.
- Shares durable settings validation with `turn/start`, including
`sandboxPolicy` plus `permissions` rejection and `serviceTier: null`
clearing.
- Emits the same settings notification when `turn/start` overrides
change the stored effective thread settings.
- Regenerates app-server protocol schema fixtures and updates
`app-server/README.md`.

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

@@ -517,6 +517,13 @@ client_request_definitions! {
         serialization: thread_id(params.thread_id),
         response: v2::ThreadMetadataUpdateResponse,
     },
+    #[experimental("thread/settings/update")]
+    ThreadSettingsUpdate => "thread/settings/update" {
+        params: v2::ThreadSettingsUpdateParams,
+        inspect_params: true,
+        serialization: thread_id(params.thread_id),
+        response: v2::ThreadSettingsUpdateResponse,
+    },
     #[experimental("thread/memoryMode/set")]
     ThreadMemoryModeSet => "thread/memoryMode/set" {
         params: v2::ThreadMemoryModeSetParams,
@@ -1470,6 +1477,8 @@ server_notification_definitions! {
     ThreadGoalUpdated => "thread/goal/updated" (v2::ThreadGoalUpdatedNotification),
     #[experimental("thread/goal/cleared")]
     ThreadGoalCleared => "thread/goal/cleared" (v2::ThreadGoalClearedNotification),
+    #[experimental("thread/settings/updated")]
+    ThreadSettingsUpdated => "thread/settings/updated" (v2::ThreadSettingsUpdatedNotification),
     ThreadTokenUsageUpdated => "thread/tokenUsage/updated" (v2::ThreadTokenUsageUpdatedNotification),
     TurnStarted => "turn/started" (v2::TurnStartedNotification),
     HookStarted => "hook/started" (v2::HookStartedNotification),
@@ -3094,6 +3103,40 @@ mod tests {
         );
     }
 
+    #[test]
+    fn thread_settings_updated_notification_is_marked_experimental() {
+        let notification =
+            ServerNotification::ThreadSettingsUpdated(v2::ThreadSettingsUpdatedNotification {
+                thread_id: "thr_123".to_string(),
+                thread_settings: v2::ThreadSettings {
+                    cwd: absolute_path("/tmp/repo"),
+                    approval_policy: v2::AskForApproval::Never,
+                    approvals_reviewer: v2::ApprovalsReviewer::User,
+                    sandbox_policy: v2::SandboxPolicy::DangerFullAccess,
+                    active_permission_profile: None,
+                    model: "gpt-5.4".to_string(),
+                    model_provider: "openai".to_string(),
+                    service_tier: None,
+                    effort: None,
+                    summary: None,
+                    collaboration_mode: codex_protocol::config_types::CollaborationMode {
+                        mode: codex_protocol::config_types::ModeKind::Default,
+                        settings: codex_protocol::config_types::Settings {
+                            model: "gpt-5.4".to_string(),
+                            reasoning_effort: None,
+                            developer_instructions: None,
+                        },
+                    },
+                    personality: None,
+                },
+            });
+
+        assert_eq!(
+            crate::experimental_api::ExperimentalApi::experimental_reason(&notification),
+            Some("thread/settings/updated")
+        );
+    }
+
     #[test]
     fn thread_realtime_started_notification_is_marked_experimental() {
         let notification =