Enable paginated thread history in app-server (#33364)

## What changed

- Allow `thread/start` to select experimental `historyMode: "paginated"` when the thread store supports turn and item pagination.
- Resume paginated threads with bounded model context and require `excludeTurns: true` instead of loading full history.
- Return `turnsBackwardsCursor` and `itemsBackwardsCursor` from `thread/resume` so clients can hydrate durable history through `thread/turns/list` and `thread/items/list` while newer records arrive through live notifications.
- Reject operations that still require full history, including `initialTurnsPage`, `thread/read` with `includeTurns: true`, `thread/rollback`, and detached review.
- Make item cursors independent of the optional `turnId` filter.

## Testing

- Cover paginated thread start, cold and live resume, backwards cursor hydration, metadata reads, unsupported operations, and stores without pagination support.

GitOrigin-RevId: c42f9b268e29d049843330f474fa3ecde12f5b8a

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

@@ -219,6 +219,8 @@ fn thread_resume_response_round_trips_initial_turns_page() {
             next_cursor: Some("cursor_next".to_string()),
             backwards_cursor: Some("cursor_back".to_string()),
         }),
+        turns_backwards_cursor: Some("turns_head".to_string()),
+        items_backwards_cursor: Some("items_head".to_string()),
     };
 
     let value = serde_json::to_value(&response).expect("serialize thread resume response");
@@ -230,6 +232,14 @@ fn thread_resume_response_round_trips_initial_turns_page() {
             "backwardsCursor": "cursor_back",
         }))
     );
+    assert_eq!(
+        value.get("turnsBackwardsCursor"),
+        Some(&json!("turns_head"))
+    );
+    assert_eq!(
+        value.get("itemsBackwardsCursor"),
+        Some(&json!("items_head"))
+    );
     let decoded = serde_json::from_value::<ThreadResumeResponse>(value)
         .expect("deserialize thread resume response");
     assert_eq!(decoded, response);

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

@@ -435,6 +435,20 @@ pub struct ThreadResumeResponse {
     #[experimental("thread/resume.initialTurnsPage")]
     #[serde(default)]
     pub initial_turns_page: Option<TurnsPage>,
+    /// Opaque head cursor for hydrating paginated turns backwards.
+    ///
+    /// Pass this as `cursor` to `thread/turns/list` with
+    /// `sortDirection: "desc"`. The first page includes the cursor's head turn.
+    #[experimental("thread/resume.turnsBackwardsCursor")]
+    #[serde(default)]
+    pub turns_backwards_cursor: Option<String>,
+    /// Opaque head cursor for hydrating paginated items backwards.
+    ///
+    /// Pass this as `cursor` to `thread/items/list` with
+    /// `sortDirection: "desc"`. The first page includes the cursor's head item.
+    #[experimental("thread/resume.itemsBackwardsCursor")]
+    #[serde(default)]
+    pub items_backwards_cursor: Option<String>,
 }
 
 impl ThreadResumeResponse {