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/src/in_process.rs

@@ -732,6 +732,7 @@ mod tests {
     use codex_app_server_protocol::ThreadStartResponse;
     use codex_app_server_protocol::Turn;
     use codex_app_server_protocol::TurnCompletedNotification;
+    use codex_app_server_protocol::TurnItemsView;
     use codex_app_server_protocol::TurnStatus;
     use codex_core::config::ConfigBuilder;
     use pretty_assertions::assert_eq;
@@ -961,6 +962,7 @@ mod tests {
                 turn: Turn {
                     id: "turn-1".to_string(),
                     items: Vec::new(),
+                    items_view: TurnItemsView::NotLoaded,
                     status: TurnStatus::Completed,
                     error: None,
                     started_at: None,