feat(app-server): thread/turns/items/list -> thread/items/list (#29705)

## Description

Rename the experimental app-server item pagination API from
`thread/turns/items/list` to `thread/items/list` and make `turnId`
optional. Clients can now page persisted items across a thread, or still
filter to one turn when needed.

## What changed

- Rename the request/response protocol types and JSON-RPC method to
`ThreadItemsList*` / `thread/items/list`.
- Pass optional `turnId` through to `ThreadStore::list_items`.
- Update app-server docs and focused protocol/app-server tests.

## Validation

- `just test -p codex-app-server-protocol thread_items_list_round_trips`
- `just test -p codex-app-server thread_items_list_returns_unsupported`

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

@@ -224,10 +224,10 @@ fn thread_resume_response_round_trips_initial_turns_page() {
 }
 
 #[test]
-fn thread_turns_items_list_round_trips() {
-    let params = ThreadTurnsItemsListParams {
+fn thread_items_list_round_trips() {
+    let params = ThreadItemsListParams {
         thread_id: "thr_123".to_string(),
-        turn_id: "turn_456".to_string(),
+        turn_id: Some("turn_456".to_string()),
         cursor: Some("cursor_1".to_string()),
         limit: Some(50),
         sort_direction: Some(SortDirection::Asc),
@@ -243,7 +243,7 @@ fn thread_turns_items_list_round_trips() {
             "sortDirection": "asc",
         })
     );
-    let response = ThreadTurnsItemsListResponse {
+    let response = ThreadItemsListResponse {
         data: vec![ThreadItem::ContextCompaction {
             id: "item_1".to_string(),
         }],
@@ -259,6 +259,32 @@ fn thread_turns_items_list_round_trips() {
             "backwardsCursor": "cursor_0",
         })
     );
+
+    let params_without_turn = ThreadItemsListParams {
+        thread_id: "thr_123".to_string(),
+        turn_id: None,
+        cursor: None,
+        limit: None,
+        sort_direction: None,
+    };
+
+    assert_eq!(
+        serde_json::to_value(&params_without_turn).expect("serialize params without turn"),
+        json!({
+            "threadId": "thr_123",
+            "turnId": null,
+            "cursor": null,
+            "limit": null,
+            "sortDirection": null,
+        })
+    );
+    assert_eq!(
+        serde_json::from_value::<ThreadItemsListParams>(json!({
+            "threadId": "thr_123",
+        }))
+        .expect("deserialize params without turn"),
+        params_without_turn
+    );
 }
 
 #[test]