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/tui/src/app/pending_interactive_replay.rs

@@ -665,6 +665,7 @@ mod tests {
             thread_id: "thread-1".to_string(),
             turn: Turn {
                 id: turn_id.to_string(),
+                items_view: codex_app_server_protocol::TurnItemsView::Full,
                 items: Vec::new(),
                 status: TurnStatus::Completed,
                 error: None,

codex-rs/tui/src/app/tests.rs

@@ -4089,6 +4089,7 @@ async fn height_shrink_schedules_resize_reflow() {
 fn test_turn(turn_id: &str, status: TurnStatus, items: Vec<ThreadItem>) -> Turn {
     Turn {
         id: turn_id.to_string(),
+        items_view: codex_app_server_protocol::TurnItemsView::Full,
         items,
         status,
         error: None,
@@ -4667,6 +4668,7 @@ async fn replay_thread_snapshot_replays_turn_history_in_order() {
             turns: vec![
                 Turn {
                     id: "turn-1".to_string(),
+                    items_view: codex_app_server_protocol::TurnItemsView::Full,
                     items: vec![ThreadItem::UserMessage {
                         id: "user-1".to_string(),
                         content: vec![AppServerUserInput::Text {
@@ -4682,6 +4684,7 @@ async fn replay_thread_snapshot_replays_turn_history_in_order() {
                 },
                 Turn {
                     id: "turn-2".to_string(),
+                    items_view: codex_app_server_protocol::TurnItemsView::Full,
                     items: vec![
                         ThreadItem::UserMessage {
                             id: "user-2".to_string(),

codex-rs/tui/src/app/thread_events.rs

@@ -364,6 +364,7 @@ mod tests {
     fn test_turn(turn_id: &str, status: TurnStatus, items: Vec<ThreadItem>) -> Turn {
         Turn {
             id: turn_id.to_string(),
+            items_view: codex_app_server_protocol::TurnItemsView::Full,
             items,
             status,
             error: None,