add turn items view to app-server turns (#21063)

## Why

`Turn.items` currently overloads an empty array to mean either that no
items exist or that the server intentionally did not load them for this
response. That ambiguity blocks future lazy-loading work where clients
need to distinguish unloaded, summary, and fully hydrated turn payloads.

## What changed

- add a new `TurnItemsView` enum with `notLoaded`, `summary`, and `full`
variants
- add required `itemsView` metadata to app-server `Turn` payloads
- mark reconstructed persisted history as `full` and live shell-style
turn payloads as `notLoaded`
- keep current `thread/turns/list` behavior unchanged and document that
it still returns `full` turns today
- regenerate the JSON and TypeScript protocol fixtures

## Verification

- `just write-app-server-schema`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server thread_read_can_include_turns`
- `cargo test -p codex-app-server
thread_turns_list_can_page_backward_and_forward`
- `cargo test -p codex-app-server
thread_resume_rejects_history_when_thread_is_running`
- `just fix -p codex-app-server-protocol`
- `just fix -p codex-app-server`
- `just fmt`

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

@@ -17,6 +17,7 @@ use crate::protocol::v2::ThreadItem;
 use crate::protocol::v2::Turn;
 use crate::protocol::v2::TurnError as V2TurnError;
 use crate::protocol::v2::TurnError;
+use crate::protocol::v2::TurnItemsView;
 use crate::protocol::v2::TurnStatus;
 use crate::protocol::v2::UserInput;
 use crate::protocol::v2::WebSearchAction;
@@ -1166,6 +1167,7 @@ impl From<PendingTurn> for Turn {
         Self {
             id: value.id,
             items: value.items,
+            items_view: TurnItemsView::Full,
             error: value.error,
             status: value.status,
             started_at: value.started_at,
@@ -1180,6 +1182,7 @@ impl From<&PendingTurn> for Turn {
         Self {
             id: value.id.clone(),
             items: value.items.clone(),
+            items_view: TurnItemsView::Full,
             error: value.error.clone(),
             status: value.status.clone(),
             started_at: value.started_at,
@@ -1453,6 +1456,7 @@ mod tests {
                 started_at: None,
                 completed_at: None,
                 duration_ms: None,
+                items_view: TurnItemsView::Full,
                 items: vec![
                     ThreadItem::UserMessage {
                         id: "item-1".into(),
@@ -2723,6 +2727,7 @@ mod tests {
                 started_at: None,
                 completed_at: None,
                 duration_ms: None,
+                items_view: TurnItemsView::Full,
                 items: Vec::new(),
             }]
         );
@@ -2982,6 +2987,7 @@ mod tests {
                 started_at: None,
                 completed_at: None,
                 duration_ms: None,
+                items_view: TurnItemsView::Full,
                 items: vec![ThreadItem::UserMessage {
                     id: "item-1".into(),
                     content: vec![UserInput::Text {

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -5434,10 +5434,11 @@ impl From<CoreTokenUsage> for TokenUsageBreakdown {
 #[ts(export_to = "v2/")]
 pub struct Turn {
     pub id: String,
-    /// Only populated on a `thread/resume` or `thread/fork` response.
-    /// For all other responses and notifications returning a Turn,
-    /// the items field will be an empty list.
+    /// Thread items currently included in this turn payload.
     pub items: Vec<ThreadItem>,
+    /// Describes how much of `items` has been loaded for this turn.
+    #[serde(default)]
+    pub items_view: TurnItemsView,
     pub status: TurnStatus,
     /// Only populated when the Turn's status is failed.
     pub error: Option<TurnError>,
@@ -5452,6 +5453,19 @@ pub struct Turn {
     pub duration_ms: Option<i64>,
 }
 
+#[derive(Default, Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub enum TurnItemsView {
+    /// `items` was not loaded for this turn. The field is intentionally empty.
+    NotLoaded,
+    /// `items` contains only a display summary for this turn.
+    Summary,
+    /// `items` contains every ThreadItem available from persisted app-server history for this turn.
+    #[default]
+    Full,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, Eq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -8441,6 +8455,22 @@ mod tests {
         }
     }
 
+    #[test]
+    fn turn_defaults_legacy_missing_items_view_to_full() {
+        let turn: Turn = serde_json::from_value(json!({
+            "id": "turn_123",
+            "items": [],
+            "status": "completed",
+            "error": null,
+            "startedAt": null,
+            "completedAt": null,
+            "durationMs": null,
+        }))
+        .expect("legacy turn should deserialize");
+
+        assert_eq!(turn.items_view, TurnItemsView::Full);
+    }
+
     #[test]
     fn thread_list_params_accepts_single_cwd() {
         let params = serde_json::from_value::<ThreadListParams>(json!({