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-client/src/lib.rs

@@ -1154,6 +1154,7 @@ mod tests {
             thread_id: "thread".to_string(),
             turn: codex_app_server_protocol::Turn {
                 id: "turn".to_string(),
+                items_view: codex_app_server_protocol::TurnItemsView::Full,
                 items: Vec::new(),
                 status: codex_app_server_protocol::TurnStatus::Completed,
                 error: None,
@@ -1984,6 +1985,7 @@ mod tests {
                         thread_id: "thread".to_string(),
                         turn: codex_app_server_protocol::Turn {
                             id: "turn".to_string(),
+                            items_view: codex_app_server_protocol::TurnItemsView::Full,
                             items: Vec::new(),
                             status: codex_app_server_protocol::TurnStatus::Completed,
                             error: None,