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/thread.rs

@@ -1316,9 +1316,11 @@ pub struct ThreadTurnsListResponse {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
-pub struct ThreadTurnsItemsListParams {
+pub struct ThreadItemsListParams {
     pub thread_id: String,
-    pub turn_id: String,
+    /// Optional turn id to filter by. When omitted, returns items across the thread.
+    #[ts(optional = nullable)]
+    pub turn_id: Option<String>,
     /// Opaque cursor to pass to the next call to continue after the last item.
     #[ts(optional = nullable)]
     pub cursor: Option<String>,
@@ -1333,7 +1335,7 @@ pub struct ThreadTurnsItemsListParams {
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
-pub struct ThreadTurnsItemsListResponse {
+pub struct ThreadItemsListResponse {
     pub data: Vec<ThreadItem>,
     /// Opaque cursor to pass to the next call to continue after the last item.
     /// if None, there are no more items to return.