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/common.rs

@@ -641,12 +641,12 @@ client_request_definitions! {
         serialization: None,
         response: v2::ThreadTurnsListResponse,
     },
-    #[experimental("thread/turns/items/list")]
-    ThreadTurnsItemsList => "thread/turns/items/list" {
-        params: v2::ThreadTurnsItemsListParams,
+    #[experimental("thread/items/list")]
+    ThreadItemsList => "thread/items/list" {
+        params: v2::ThreadItemsListParams,
         // Explicitly concurrent: this primarily reads append-only rollout storage.
         serialization: None,
-        response: v2::ThreadTurnsItemsListResponse,
+        response: v2::ThreadItemsListResponse,
     },
     /// Append raw Responses API items to the thread history without starting a user turn.
     ThreadInjectItems => "thread/inject_items" {
@@ -2090,17 +2090,17 @@ mod tests {
         };
         assert_eq!(thread_turns_list.serialization_scope(), None);
 
-        let thread_turns_items_list = ClientRequest::ThreadTurnsItemsList {
+        let thread_items_list = ClientRequest::ThreadItemsList {
             request_id: request_id(),
-            params: v2::ThreadTurnsItemsListParams {
+            params: v2::ThreadItemsListParams {
                 thread_id: "thread-1".to_string(),
-                turn_id: "turn-1".to_string(),
+                turn_id: None,
                 cursor: None,
                 limit: None,
                 sort_direction: None,
             },
         };
-        assert_eq!(thread_turns_items_list.serialization_scope(), None);
+        assert_eq!(thread_items_list.serialization_scope(), None);
 
         let mcp_resource_read = ClientRequest::McpResourceRead {
             request_id: request_id(),