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/v2/thread.rs

@@ -11,7 +11,9 @@ use super::TurnEnvironmentParams;
 use super::TurnItemsView;
 use super::shared::v2_enum_from_core;
 use codex_experimental_api_macros::ExperimentalApi;
+use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::Personality;
+use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::ThreadGoalStatus as CoreThreadGoalStatus;
@@ -219,6 +221,93 @@ pub struct ThreadStartResponse {
     pub reasoning_effort: Option<ReasoningEffort>,
 }
 
+#[derive(
+    Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS, ExperimentalApi,
+)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadSettingsUpdateParams {
+    pub thread_id: String,
+    /// Override the working directory for subsequent turns.
+    #[ts(optional = nullable)]
+    pub cwd: Option<PathBuf>,
+    /// Override the approval policy for subsequent turns.
+    #[experimental(nested)]
+    #[ts(optional = nullable)]
+    pub approval_policy: Option<AskForApproval>,
+    /// Override where approval requests are routed for subsequent turns.
+    #[ts(optional = nullable)]
+    pub approvals_reviewer: Option<ApprovalsReviewer>,
+    /// Override the sandbox policy for subsequent turns.
+    #[ts(optional = nullable)]
+    pub sandbox_policy: Option<SandboxPolicy>,
+    /// Select a named permissions profile id for subsequent turns. Cannot be
+    /// combined with `sandboxPolicy`.
+    #[experimental("thread/settings/update.permissions")]
+    #[ts(optional = nullable)]
+    pub permissions: Option<String>,
+    /// Override the model for subsequent turns.
+    #[ts(optional = nullable)]
+    pub model: Option<String>,
+    /// Override the service tier for subsequent turns. `null` clears the
+    /// current service tier; omission leaves it unchanged.
+    #[serde(
+        default,
+        deserialize_with = "crate::protocol::serde_helpers::deserialize_double_option",
+        serialize_with = "crate::protocol::serde_helpers::serialize_double_option",
+        skip_serializing_if = "Option::is_none"
+    )]
+    #[ts(optional = nullable)]
+    pub service_tier: Option<Option<String>>,
+    /// Override the reasoning effort for subsequent turns.
+    #[ts(optional = nullable)]
+    pub effort: Option<ReasoningEffort>,
+    /// Override the reasoning summary for subsequent turns.
+    #[ts(optional = nullable)]
+    pub summary: Option<ReasoningSummary>,
+    /// EXPERIMENTAL - Set a pre-set collaboration mode for subsequent turns.
+    ///
+    /// For `collaboration_mode.settings.developer_instructions`, `null` means
+    /// "use the built-in instructions for the selected mode".
+    #[experimental("thread/settings/update.collaborationMode")]
+    #[ts(optional = nullable)]
+    pub collaboration_mode: Option<CollaborationMode>,
+    /// Override the personality for subsequent turns.
+    #[ts(optional = nullable)]
+    pub personality: Option<Personality>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadSettingsUpdateResponse {}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadSettings {
+    pub cwd: AbsolutePathBuf,
+    pub approval_policy: AskForApproval,
+    pub approvals_reviewer: ApprovalsReviewer,
+    pub sandbox_policy: SandboxPolicy,
+    pub active_permission_profile: Option<ActivePermissionProfile>,
+    pub model: String,
+    pub model_provider: String,
+    pub service_tier: Option<String>,
+    pub effort: Option<ReasoningEffort>,
+    pub summary: Option<ReasoningSummary>,
+    pub collaboration_mode: CollaborationMode,
+    pub personality: Option<Personality>,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct ThreadSettingsUpdatedNotification {
+    pub thread_id: String,
+    pub thread_settings: ThreadSettings,
+}
+
 #[derive(
     Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS, ExperimentalApi,
 )]