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/analytics/src/analytics_client_tests.rs

@@ -246,6 +246,7 @@ fn sample_turn_start_response(turn_id: &str) -> ClientResponsePayload {
     ClientResponsePayload::TurnStart(codex_app_server_protocol::TurnStartResponse {
         turn: Turn {
             id: turn_id.to_string(),
+            items_view: codex_app_server_protocol::TurnItemsView::Full,
             items: vec![],
             status: AppServerTurnStatus::InProgress,
             error: None,
@@ -261,6 +262,7 @@ fn sample_turn_started_notification(thread_id: &str, turn_id: &str) -> ServerNot
         thread_id: thread_id.to_string(),
         turn: Turn {
             id: turn_id.to_string(),
+            items_view: codex_app_server_protocol::TurnItemsView::Full,
             items: vec![],
             status: AppServerTurnStatus::InProgress,
             error: None,
@@ -295,6 +297,7 @@ fn sample_turn_completed_notification(
         thread_id: thread_id.to_string(),
         turn: Turn {
             id: turn_id.to_string(),
+            items_view: codex_app_server_protocol::TurnItemsView::Full,
             items: vec![],
             status,
             error: codex_error_info.map(|codex_error_info| AppServerTurnError {